Remove posh jumper during exercise (6)
I believe the answer is:
uproot
'remove' is the definition.
(I've seen this before)
'posh jumper during exercise' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'jumper' becomes 'roo' (I've seen this before).
'during' means one lot of letters goes inside another.
'exercise' becomes 'pt'.
'roo' placed into 'pt' is 'proot'.
'u'+'proot'='UPROOT'
(Other definitions for uproot that I've seen before include "Tear from ground or force to leave home" , "Tear up from natural location, eradicate" , "Completely destroy" , "Remove forcibly from a homeland to a foreign environment" , "Extirpate" .)