Gather a crowd (5)
I believe the answer is:
amass
'gather' is the definition.
(amass can mean to collect or gather)
'a crowd' is the wordplay.
'crowd' becomes 'mass' (massing is a kind of crowding).
'a'+'mass'='AMASS'
(Other definitions for amass that I've seen before include "Accumulate, heap up" , "Stock up on" , "Get together" , "Stockpile" , "Gathers" .)