Perhaps who e.g. mum favouring old sister hugs (8,7)
I believe the answer is:
relative pronoun
'perhaps who' is the definition.
(I know that who is a relative pronoun)
'e g mum favouring old sister hugs' is the wordplay.
'e g mum' becomes 'relative' (mother is a type of relative).
'favouring' becomes 'pro' (being in favour of something).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sister' becomes 'nun' (sister is a term of address for a nun).
'hugs' means one lot of letters goes inside another.
'o' going into 'nun' is 'noun'.
'relative'+'pro'+'noun'='RELATIVE PRONOUN'
(Other definitions for relative pronoun that I've seen before include "Maybe what" , "Say which" , "It could be that" .)