Power and sex will be mine! (3)
I believe the answer is:
pit
'mine' is the definition.
(down the pit)
'power and sex' is the wordplay.
'power' becomes 'p' (symbol in physics).
'and' means one lot of letters go next to another.
'sex' becomes 'it' (a common euphemism).
'p'+'it'='PIT'
'will be' is the link.
(Other definitions for pit that I've seen before include "Quarry" , "Sizeable hole in the ground" , "Stop to refuel" , "Mining hole" , "See 9" .)