Hard worker succeeds on a barrier (7)
I believe the answer is:
adamant
'hard' is the definition.
(I've seen this before)
'worker succeeds on a barrier' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'succeeds' says to put letters next to each other (I've seen this in other clues).
'on' says to put letters next to each other.
'barrier' becomes 'dam' (dam is a kind of barrier).
'a' next to 'dam' is 'adam'.
'ant' after 'adam' is 'ADAMANT'.
(Other definitions for adamant that I've seen before include "Refusing to budge" , "Hard stone" , "Firmly unchanging" , "Inflexible: fabulously hard rock" , "Positive or intransigent" .)