Constables not entirely competent (4)
I believe the answer is:
able
'competent' is the definition.
(similar in meaning)
'constables not entirely' is the wordplay.
'not entirely' indicates a hidden word.
'ABLE' is hidden within 'constables'.
(Other definitions for able that I've seen before include "Fitted" , "Proficient; skilful" , "Having considerable skill" , "Capable or proficient" , "Fully competent" .)