Muscle priest strained carrying lid of casket (7)
I believe the answer is:
triceps
'muscle' is the definition.
(triceps is a kind of muscle)
'priest strained carrying lid of casket' is the wordplay.
'strained' is an anagram indicator.
'carrying' means one lot of letters goes inside another.
'lid of' suggests taking the first letters (in a down clue, the lid could be the top letter).
The first letter of 'casket' is 'c'.
'priest' anagrammed gives 'trieps'.
'trieps' placed around 'c' is 'TRICEPS'.
(Other definitions for triceps that I've seen before include "One stretches out an arm!" , "Muscle that extends the forearm" , "Extensor muscle" , "Source of uplifting strength" , "Muscle at the back of the upper arm" .)