Collect a mothership? (5)

I believe the answer is:

amass

I'm a little stuck... Click here to teach me more about this clue!

'collect' is the definition.
(synonyms)

'a mothership?' is the wordplay.
'mothership?' becomes 'mass' (I can't justify this - if you can you should believe this answer much more).
'a'+'mass'='AMASS'

Can you help me to learn more?

(Other definitions for amass that I've seen before include "Stock up on" , "Get together" , "Accumulate, heap up" , "Assemble" , "store" .)

I've seen this clue in The Mirror.

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