Fruit drink that's pink I had knocked back inside (7)
I believe the answer is:
cordial
'fruit drink' is the definition.
(I've seen this before)
'pink i had knocked back inside' is the wordplay.
'pink' becomes 'coral' (coral is a kind of pink).
'i had' becomes 'id' (I'd).
'knocked back' shows that the letters should be reversed in order.
'inside' means one lot of letters goes inside another.
'id' in reverse letter order is 'di'.
'coral' placed around 'di' is 'CORDIAL'.
'that's' is the link.
(Other definitions for cordial that I've seen before include "Affable type of sweet drink?" , "stimulating drink" , "Pleasant - soft drink" , "Friendly; fruit drink" , "Hearty - drink" .)