Garment starts off small and rarely increases (4)
I believe the answer is:
sari
'garment' is the definition.
(sari is a kind of garment)
'starts off small and rarely increases' is the wordplay.
'starts off' says to take the initial letters.
The initial letters of 'small and rarely increases' is 'SARI'.
(Other definitions for sari that I've seen before include "South Asian garment (variant spelling)" , "Asian dress" , "Traditional dress of Indian women" , "Indian robe" , "Hindu woman's dress" .)