A bogus RAF choir assembled, admitting right number (11,4)
I believe the answer is:
scarborough fair
'number' is the definition.
(a song or musical number)
'a bogus raf choir assembled admitting right' is the wordplay.
'assembled' is an anagram indicator.
'admitting' means one lot of letters goes inside another.
'right' becomes 'r' (common abbreviation).
'a'+'bogus'+'raf'+'choir'='abogusrafchoir'
'abogusrafchoir' with letters rearranged gives 'scarbooughfair'.
'scarbooughfair' enclosing 'r' is 'SCARBOROUGH FAIR'.
(Other definitions for scarborough fair that I've seen before include "number" , "Song" , "1960s pop" , "Alloy of mercury; blend" .)