Gather mother's upset by idiot (5)
I believe the answer is:
amass
'gather' is the definition.
('amass' can be a synonym of 'gather')
'mother's upset by idiot' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'upset' shows that the letters should be reversed in order.
'by' says to put letters next to each other.
'idiot' becomes 'ass' (I've seen this before**).
'ma' reversed gives 'am'.
'am'+'ass'='AMASS'
(Other definitions for amass that I've seen before include "Accumulate over time" , "Stockpile" , "Collect, hoard" , "Accumulate, pile up" , "run up?" .)