Posh princess uses mains leads for element (7)
I believe the answer is:
uranium
'for element' is the definition.
(uranium is a kind of element)
'posh princess uses mains leads' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'princess' becomes 'rani' (I've seen this before).
'leads' suggests taking the first letters.
The initial letters of 'uses mains' is 'um'.
'u'+'rani'+'um'='URANIUM'
(Other definitions for uranium that I've seen before include "Source of power" , "Radioactive metallic element" , "Chemical element, source of nuclear energy" , "Radioactive element of atomic number 92" , "Fissile element" .)