Resolute, a worker crossing barrier (7)
I believe the answer is:
adamant
'resolute' is the definition.
(I've seen this before)
'a worker crossing barrier' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'crossing' is an insertion indicator.
'barrier' becomes 'dam' (dam is a kind of barrier).
'a'+'ant'='aant'
'aant' going around 'dam' is 'ADAMANT'.
(Other definitions for adamant that I've seen before include "Decided" , "intractable" , "Completely determined" , "Resolute; diamond (arch.)" , "Obdurate and unyielding" .)