Green’s Dictionary of Slang

(Aus./US) committed, reliable, e.g. a straight-ahead guy.

1888		‘Rolf Boldrewood’ Robbery Under Arms (1922) 122: It ain’t so easy to get a regular up-and-down straight-ahead jury in a place of this sort.
1947		P. Newton Wayleggo (1953) 90: He was just a straight ahead bucker with no cunning tricks.
1990		J. Wambaugh Golden Orange (1991) 58: You’re a straight-ahead guy.
2000		Guardian Rev. 21 Apr. 20: This description might well ring a bell with some straightahead jazz listeners.
2013		B. Leopold ‘With One Stone’ in ThugLit Apr. [ebook] ‘He’s no crack addict, or juicer, or pill-popper; he’s a straight-ahead con man’.