Green’s Dictionary of Slang

mod adj.

[abbr. SE modern]

fashionable, up-to-date.

[Aus]‘Charles Barrett’ Address: Kings Cross 70: These were the socialists from the North Shore, with Tom Jones coiffures, big black bows on the backs of their heads and mod. fashions – the Way Out long skirts and be-frilled and be-ribboned blouses.
[US]H. Sebold Adolescence 222: In early 1966 [...] Dayton’s Department Store brought the top English designer of Mod Clothing, John Stephen, to Minneapolis to open a special shop.
[US]J. Wambaugh Choirboys (1976) 290: Spencer’s so mod he wears flared jockey shorts.
[US]E. Folb Runnin’ Down Some Lines 48: He ain’t mod, he ain’ dress mod, comfortable, nothin’.

In compounds

mod squad (n.)

plainclothes police, usu. young and dressed in the prevailing teenage and early 20s fashions, who look for crime in colleges and local youth centres.

[US]E. Folb Runnin’ Down Some Lines 67: You find expressions like [...] mod squad for young undercover officers.

In phrases