In theory, no friend receives maidens at home (9)
I believe the answer is:
nominally
'in theory' is the definition.
(I've seen this before)
'no friend receives maidens at home' is the wordplay.
'friend' becomes 'ally' (an ally is a friend or supporter).
'receives' indicates putting letters inside (inserted letters are received).
'maidens' becomes 'm' (cricket abbreviation).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'no'+'ally'='noally'
'm'+'in'='min'
'noally' enclosing 'min' is 'NOMINALLY'.
(Other definitions for nominally that I've seen before include "In theory" , "In name only" , "Theoretically" .)