Southern wine brewed for strength (5)
I believe the answer is:
'strength' is the definition.
(sinew is a kind of strength)
'southern wine brewed' is the wordplay.
'southern' becomes 's' (abbreviation).
'brewed' is an anagram indicator.
'wine' with letters rearranged gives 'inew'.
'for' acts as a link.
(Other definitions for sinew that I've seen before include "source of physical strength", "As synonym for 'strength'", "Tendon, for example", "power source", "Inelastic tissue".)