Always a feature of transitive verbs (4)
I believe the answer is:
ever
'always' is the definition.
(synonyms)
'a feature of transitive verbs' is the wordplay.
'a feature of' says the answer is hidden in the clue.
'EVER' is hidden within 'transitive verbs'.
(Other definitions for ever that I've seen before include "Continuously" , "Perpetually" , "Eternally" , "Possibly" , "Always and permanent" .)