Fit possibly to pass muster initially? (5)
I believe the answer is:
spasm
'fit' is the definition.
(thesaurus)
'possibly to pass muster initially?' is the wordplay.
'possibly' indicates anagramming the letters.
'muster initially?' becomes 'm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pass' is an anagram of 'spas'.
'spas'+'m'='SPASM'
Can you help me to learn more?
(Other definitions for spasm that I've seen before include "Muscular convulsion" , "Convulsion or paroxysm" , "cramp" , "Involuntary muscular movement" , "Fit" .)