Conservative barrister, not old, is to screw up (7)
I believe the answer is:
crumple
'is to screw up' is the definition.
'conservative barrister not old' is the wordplay.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'barrister' becomes 'rumpole' (ref to TV show Rumpole of the Bailey).
'not' suggests deleting specific letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rumpole' with 'o' taken out is 'rumple'.
'c'+'rumple'='CRUMPLE'
(Other definitions for crumple that I've seen before include "Pucker" , "Go to pieces" , "wrinkle" , "Make wrinkled" , "Collapse, cave in" .)