Grand pound shop's opening for pants (8)
I believe the answer is:
knickers
'pants' is the definition.
(knickers is a kind of pants)
'grand pound shop's opening' is the wordplay.
'grand' becomes 'k' ('K' can be a synonym of 'grand').
'pound' becomes 'nicker' (I've seen this before).
'opening' suggests taking the first letters.
The first letter of 'shops' is 's'.
'k'+'nicker'+'s'='KNICKERS'
'for' acts as a link.
(Other definitions for knickers that I've seen before include "bother" , "Female undergarment" , "Mild expletive" , "Elasticated undergarment" , "Drat!" .)