The top of carpet pile must be tacky (5)
I believe the answer is:
cheap
'tacky' is the definition.
('cheap' can be a synonym of 'tacky')
'the top of carpet pile' is the wordplay.
'the top of' suggests taking the first letters.
'pile' becomes 'heap' ('heap' can be a synonym of 'pile'**).
The initial letter of 'carpet' is 'c'.
'c'+'heap'='CHEAP'
'must be' is the link.
(Other definitions for cheap that I've seen before include "Second-rate" , "Budget -cheesy" , "Shoddy" , "bargain" , "A snip" .)