The first man, a worker, is unshakeable (7)
I believe the answer is:
adamant
'unshakeable' is the definition.
(I've seen this before)
'the first man a worker' is the wordplay.
'the first' is a charade indicator (letters next to each other).
'man' becomes 'adam' (Adam is an example).
'a worker' becomes 'ant' (worker is a type of ant).
'adam'+'ant' is 'ADAMANT'.
'is' is the link.
(Other definitions for adamant that I've seen before include "Fully determined" , "Completely determined" , "Refusing to change opinion" , "Definite and unyielding" , "unshakeable in purpose" .)