By contrast, the little knight may have an advantage (4,5)
I believe the answer is:
then again
'by contrast' is the definition.
('then again' can introduce a contrasting view)
'the little knight may have an advantage' is the wordplay.
'little knight' becomes 'N' (chess abbreviation - a 'little' form).
'may have' means one lot of letters go next to another.
'an' becomes 'a'.
'advantage' becomes 'gain' (gain is a kind of advantage).
'the'+'n'+'a'+'gain'='THEN AGAIN'
(Other definitions for then again that I've seen before include "still" , "From another point of view" , "however" .)