Heads off while keeping Romeo in check (6)
I believe the answer is:
averts
'heads off' is the definition.
('avert' can be a synonym of 'head off')
'while keeping romeo in check' is the wordplay.
'while' becomes 'as'.
'keeping' indicates putting letters inside.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' is an insertion indicator.
'check' becomes 'vet' (I've seen this before**).
'r' placed into 'vet' is 'vert'.
'as' placed around 'vert' is 'AVERTS'.
(Other definitions for averts that I've seen before include "Wards off" , "Deflects, heads off" , "Stops" , "Turns aside" , "Of eyes or gaze, turns away" .)