When mother's inside store (5)
I believe the answer is:
amass
'store' is the definition.
(amassing is a kind of storing)
'when mother's inside' is the wordplay.
'when' becomes 'as'.
'mother's' becomes 'mas' (ma is a kind of mother).
'inside' is an insertion indicator.
'as' enclosing 'mas' is 'AMASS'.
(Other definitions for amass that I've seen before include "Stockpile" , "store" , "Be a compiler" , "Sodium chloride" , "Accrue, heap up" .)