Garment long that covers first-class behind (8)
I believe the answer is:
pinafore
'garment' is the definition.
(pinafore is a kind of garment)
'long that covers first-class behind' is the wordplay.
'long' becomes 'pine' (to pine for someone is to long for them).
'that covers' indicates putting letters inside.
'first class' becomes 'A' (as in A-list, A-team etc).
'behind' becomes 'for' (to be 'for' something is to be behind it).
'a'+'for'='afor'
'pine' enclosing 'afor' is 'PINAFORE'.
(Other definitions for pinafore that I've seen before include "It protects female worker" , "gives homely protection" , "Sleeveless dress or apron" , "Sleeveless apron-like dress" , "Gilbert & Sullivan operetta 'HMS ........'" .)