Gather there's a mother ship (5)
I believe the answer is:
amass
'gather' is the definition.
('amass' can be a synonym of 'gather')
'there's a mother ship' is the wordplay.
'there's' is a charade indicator (letters next to each other).
'mother' becomes 'ma' (ma is a kind of mother).
'ship' becomes 'ss'.
'a'+'ma' is 'ama'.
'ama'+'ss'='AMASS'
(Other definitions for amass that I've seen before include "Put together" , "Accrue, heap up" , "Assemble" , "Accumulate over time" , "Accumulate, heap up" .)