Collect those in Mount Ossa mad to return (5)
I believe the answer is:
amass
'collect' is the definition.
('amass' can be a synonym of 'collect')
'in mount ossa mad to return' is the wordplay.
'in' indicates the answer is hidden within the clue.
'to return' says the letters should be written backwards.
'ssama' can be found hidden inside 'mount ossa mad'.
'ssama' written backwards gives 'AMASS'.
'those' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for amass that I've seen before include "Gathers" , "Accumulate, heap up" , "store" , "Collect, hoard" , "Stockpile" .)