Grab Tom for a companion (5)
I believe the answer is:
catch
'grab' is the definition.
(synonyms)
'tom for a companion' is the wordplay.
'tom' becomes 'cat' (I have seen 'A tom ' mean 'cat' so perhaps 'tom' could also mean 'cat').
'for' is a charade indicator (letters next to each other).
'a companion' becomes 'ch' (Companion of Honour).
'cat'+'ch'='CATCH'
(Other definitions for catch that I've seen before include "Fly in the ointment" , "Trap - hear - contract" , "Disadvantage that's hidden" , "Good match" , "Ensnare" .)