Gather a great deal (5)
I believe the answer is:
amass
'gather' is the definition.
(synonyms)
'a great deal' is the wordplay.
'great deal' becomes 'mass' (synonyms).
'a'+'mass'='AMASS'
(Other definitions for amass that I've seen before include "Accumulate, pile up" , "Collect, hoard" , "Assemble" , "Get together" , "Collect or gather" .)