Lock up rogue, no gentleman on reflection (8)
I believe the answer is:
imprison
'lock up' is the definition.
(I've seen this before)
'rogue no gentleman on reflection' is the wordplay.
'rogue' becomes 'imp'.
'gentleman' becomes 'sir' (I've seen this before).
'on reflection' says the letters should be written in reverse.
'no'+'sir'='nosir'
'nosir' backwards is 'rison'.
'imp'+'rison'='IMPRISON'
(Other definitions for imprison that I've seen before include "Send down" , "Place in custody" , "Put in gaol" , "shut up" , "Put behind bars" .)