Green’s Dictionary of Slang

pension n.

(UK Und.) that sum of money paid over for ‘protection’.

[UK]P. Fordham Inside the Und. 138: Small clubs [...] whose ‘pension’ was fixed on a percentage of the takings.

In phrases

on a pension

(UK Und.) used of a police officer receiving regular bribes.

[UK]D. Powis Signs of Crime 195: On the pension A person receiving regular bribes or blackmail payments, acting as a ‘leech’.