Power mindless Republican held, looking back, is worry (7)
I believe the answer is:
perturb
'worry' is the definition.
(I've seen this before)
'power mindless republican held looking back' is the wordplay.
'power' becomes 'p'.
'mindless' becomes 'brute'.
'republican' becomes 'r' (abbreviation).
'held' is an insertion indicator.
'looking back' shows that the letters should be reversed in order.
'brute' going around 'r' is 'brutre'.
'brutre' written backwards gives 'erturb'.
'p'+'erturb'='PERTURB'
'is' is the link.
(Other definitions for perturb that I've seen before include "Unsettle; PR brute (anag.)" , "upset" , "Disconcert" , "Worry" , "Make uneasy or cause to be worried" .)