Trick a pet companion (5)
I believe the answer is:
catch
'trick' is the definition.
(I've seen this in another clue)
'a pet companion' is the wordplay.
'a pet' becomes 'cat' (I've seen this in another clue).
'companion' becomes 'ch' (Companion of Honour).
'cat'+'ch'='CATCH'
(Other definitions for catch that I've seen before include "secure" , "Capture; hidden problem" , "Snag - trap" , "Intercept and hold" , "Getting round" .)