Squadron leader breaks rules and quits (7)
I believe the answer is:
resigns
'quits' is the definition.
(resigning is a kind of quitting)
'squadron leader breaks rules' is the wordplay.
'leader' indicates taking the first letters.
'breaks' means one lot of letters goes inside another (inserted letters break into the word).
'rules' becomes 'reigns' (reign can mean to rule or govern).
The initial letter of 'squadron' is 's'.
's' going into 'reigns' is 'RESIGNS'.
'and' acts as a link.
(Other definitions for resigns that I've seen before include "Leaves job voluntarily" , "abdicates" , "Gives up (a post)" , "does the honourable thing" .)