Start work on the remainder (with a wrench?) (5)
I believe the answer is:
wrest
'a wrench?' is the definition.
(I've seen this before)
'start work on the remainder' is the wordplay.
'start' says to take the initial letters.
'on' is a charade indicator (letters next to each other).
'the remainder' becomes 'rest' (synonyms).
The first letter of 'work' is 'w'.
'w'+'rest'='WREST'
'with' acts as a link.
(Other definitions for wrest that I've seen before include "With difficulty obtain" , "Seize by forceful means" , "Wrench away" , "Strew (anag.)" , "Forcibly remove (from another's possession)" .)