Dock worker to wear down old soldiers heading north (9)
I believe the answer is:
stevedore
'dock worker' is the definition.
(synonyms)
'wear down old soldiers heading north' is the wordplay.
'wear down' becomes 'erode' (I've seen this before).
'old soldiers' becomes 'vets' (short for veterans).
'heading north' is a reversal indicator.
'erode'+'vets'='erodevets'
'erodevets' backwards is 'STEVEDORE'.
'to' acts as a link.
(Other definitions for stevedore that I've seen before include "One loads and unloads ships" , "Dock worker" , "he'll have loads on board" , "one working in docks" , "Docker" .)