A fellow seizes one violently (5)
I believe the answer is:
amain
'violently' is the definition.
(archaic adverb meaning powerfully or greatly)
'a fellow seizes one' is the wordplay.
'fellow' becomes 'man' (I've seen this before).
'seizes' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'man' enclosing 'i' is 'main'.
'a'+'main'='AMAIN'
(Other definitions for amain that I've seen before include "violently" , "at great speed" , "showing great strength in poetry" , "in great haste" , "with violence" .)