Muscle function beginning to weaken (5)
I believe the answer is:
sinew
'muscle' is the definition.
(I know that muscle can be written as sinew)
'function beginning to weaken' is the wordplay.
'function' becomes 'sine' (mathematical function).
'beginning to' indicates taking the first letters.
The initial letter of 'weaken' is 'w'.
'sine'+'w'='SINEW'
(Other definitions for sinew that I've seen before include "As synonym for 'strength'" , "fibre" , "Gristle" , "Inelastic tissue" , "Tough material" .)