Noble gem loses front (4)
I believe the answer is:
earl
'noble' is the definition.
(earl is a kind of noble)
'gem loses front' is the wordplay.
'gem' becomes 'pearl' (pearl is a kind of gem).
'loses front' means to remove the first letter (I've seen 'losing front' mean this).
'pearl' with its initial letter taken off is 'EARL'.
(Other definitions for earl that I've seen before include "Noble title" , "A real peer" , "... Grey, fragrant Oriental tea" , "Titled man" , "UK nobleman ranking between marquess and viscount" .)