Member locked up with resistance (7)
I believe the answer is:
insider
'member' is the definition.
(member of a group)
'locked up with resistance' is the wordplay.
'locked up' becomes 'inside' (I've seen this before).
'with' says to put letters next to each other.
'resistance' becomes 'r' (electronics abbreviation).
'inside'+'r'='INSIDER'
(Other definitions for insider that I've seen before include "Possible mole" , "One with special knowledge" , "He's in the circle, knows the right people" , "I may know secrets" , "one familiar with company dealings?" .)