Weight plummeting in pig's fibrous tissue (5)
I believe the answer is:
sinew
'fibrous tissue' is the definition.
(sinew is a kind of fibrous tissue)
'weight plummeting in pig's' is the wordplay.
I cannot quite see how this works, but
'weight' could be 'w' (short for weight) and 'w' is located in the answer.
'in' is present in the answer.
The remaining letters 'se' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for sinew that I've seen before include "Gristle" , "Tough material" , "Force" , "Tendon, for example" , "Tough connective tissue" .)