Whips bloke boxing one with conviction (8)
I believe the answer is:
flagella
'whips' is the definition.
(I've seen this before)
'bloke boxing one with conviction' is the wordplay.
'bloke' becomes 'fella' (non-standard spelling of 'fellow').
'boxing' is an insertion indicator ('box' can mean contain, as in to put in a box).
'one with conviction' becomes 'lag' (lag can informally mean a convict).
'fella' going around 'lag' is 'FLAGELLA'.
(Other definitions for flagella that I've seen before include "whips" , "Runners" .)