Row concerning sex from behind? (4)
I believe the answer is:
tier
'row' is the definition.
(a tier is a row of things)
'concerning sex from behind?' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'sex' becomes 'it' (common euphemism).
'from behind?' shows that the letters should be reversed in order (I've seen 'from behind' mean this).
're'+'it'='reit'
'reit' in reverse letter order is 'TIER'.
(Other definitions for tier that I've seen before include "Storey" , "Hierarchical level" , "band" , "range" , "Person joining" .)