Pig dropping weight, turning to muscle (5)
I believe the answer is:
sinew
'to muscle' is the definition.
('sinew' can be a synonym of 'muscle')
'pig dropping weight' is the wordplay.
'pig' becomes 'sine' (I am not sure about this - if you are sure you should believe this answer much more).
'dropping' suggests taking the first letters (I've seen 'drop of' mean this).
The initial letter of 'weight' is 'w'.
'sine'+'w'='SINEW'
'turning' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sinew that I've seen before include "fibre" , "Tendon - ligament" , "Cord of tissue connecting muscle to bone" , "Tough material" , "Gristle" .)