Introduction of competition hurt store (5)
I believe the answer is:
cache
'store' is the definition.
(cache is a kind of store)
'introduction of competition hurt' is the wordplay.
'introduction of' says to take the initial letters.
'hurt' becomes 'ache' ('ache' can be a synonym of 'hurt'**).
The initial letter of 'competition' is 'c'.
'c'+'ache'='CACHE'
(Other definitions for cache that I've seen before include "Hidden storage place" , "Stockpile" , "Secret store of valuables" , "Collection of hidden things" , "Secret store, hoard" .)