Summon up (6)
I believe the answer is:
'summon up' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for muster that I've seen before include "Assemble for duty" , "a lot of peacocks" , "assembled troops?" , "Bring (troops) together" , "Set rum badly for the calling together of the troops" .)