Meant to get 20% off? It's free! (8)
I believe the answer is:
liberate
'free' is the definition.
(liberating is a kind of freeing)
'meant to get 20 off? it's' is the wordplay.
I cannot quite understand how this works, but
'get' could be 'be' ('get' can mean 'come to be') and 'be' is found within the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for liberate that I've seen before include "Set free, as from oppressive authority" , "Deliver" , "Make free" , "Free (someone)" , "let go" .)