Resolute, a worker crossing weir (7)
I believe the answer is:
adamant
'resolute' is the definition.
(I've seen this before)
'a worker crossing weir' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'crossing' is an insertion indicator.
'weir' becomes 'dam' (I've seen this before).
'a'+'ant'='aant'
'aant' going around 'dam' is 'ADAMANT'.
(Other definitions for adamant that I've seen before include "popstar" , "Unyielding to requests, stubborn" , "Refusing to change one's mind" , "Inexorable" , "Decided" .)