Superior muscle turned to flab at the end (5)
I believe the answer is:
abbot
'superior' is the definition.
(abbot is a kind of superior)
'muscle turned to flab at the end' is the wordplay.
'muscle' becomes 'ab' (short for abdominal muscle).
'turned' says the letters should be written in reverse.
'at the end' says to take the final letters.
The final letter of 'flab' is 'b'.
'to'+'b'='tob'
'tob' written backwards gives 'bot'.
'ab'+'bot'='ABBOT'
(Other definitions for abbot that I've seen before include "Monastic chief" , "Father" , "Head of a community of monks" , "friar" , "Monks' superior" .)