Eager to be surviving, full of muscle (9)
I believe the answer is:
expectant
'eager' is the definition.
(thesaurus)
'surviving full of muscle' is the wordplay.
'surviving' becomes 'extant' ('extant' can be similar in meaning to 'surviving').
'full of' indicates putting letters inside.
'muscle' becomes 'pec' (I've seen this in other clues).
'extant' going around 'pec' is 'EXPECTANT'.
'to be' is the link.
(Other definitions for expectant that I've seen before include "Hopeful" , "Anticipating" , "Eager to see what happens" , "Agog" , "Waiting for something" .)