Rock old lady over sex with sweetheart (7)
I believe the answer is:
granite
'rock' is the definition.
(granite is a kind of rock)
'old lady over sex with sweetheart' is the wordplay.
'old lady' becomes 'gran' (I've seen this before**).
'over' is a charade indicator (letters next to each other).
'sex' becomes 'it' (a common euphemism).
'with' is a charade indicator (letters next to each other).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'gran'+'it'+'e'='GRANITE'
(Other definitions for granite that I've seen before include "Tangier (anag.)" , "determination" , "Igneous rock, generally composed of feldspar and mica and quartz" , "Constituent in Dartmoor" , "Very hard type of rock" .)