It's starting to feel thin and limp on the side (5)
I believe the answer is:
'limp on the side' is the definition.
'flank' can be an answer for 'side' (I've seen this in another clue). I am unsure of the 'limp on the' bit.
'it's starting to feel thin' is the wordplay.
'it's starting to' indicates taking the first letters.
'thin' becomes 'lank' (similar in meaning).
The first letter of 'feel' is 'f'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for flank that I've seen before include "Side of an animal" , "Haunch" .)