A worker crossing barrier is very determined (7)
I believe the answer is:
adamant
'very determined' is the definition.
(I've seen this in another clue)
'a worker crossing barrier' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'crossing' indicates putting letters inside.
'barrier' becomes 'dam' (dam is a kind of barrier).
'a'+'ant'='aant'
'aant' enclosing 'dam' is 'ADAMANT'.
'is' is the link.
(Other definitions for adamant that I've seen before include "Positive or intransigent" , "Definite and unyielding" , "Inflexible: fabulously hard rock" , "Refusing to change opinion" , "Obdurate and unyielding" .)