Bank on relations turning up (4)
I believe the answer is:
tier
'bank' is the definition.
(both can mean a row of similar things)
'on relations turning up' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'relations' becomes 'it' (both can mean sex).
'turning up' says the letters should be written in reverse.
're'+'it'='reit'
'reit' reversed gives 'TIER'.
(Other definitions for tier that I've seen before include "'Level, grade (4)'" , "Row - one forming a knot" , "Row in theatre" , "Row, layer" , "Rite (anag.)" .)