Berceuse in two keys penned by French composer (7)
I believe the answer is:
lullaby
'berceuse' is the definition.
(berceuse is a French term for a lullaby)
'two keys penned by french composer' is the wordplay.
'two keys' becomes 'AB' (eg A major and B major - musical keys).
'penned by' is an insertion indicator (inserted letters are penned in).
'french composer' becomes 'lully' (I've seen this before).
'ab' inserted inside 'lully' is 'LULLABY'.
'in' is the link.
(Other definitions for lullaby that I've seen before include "Soothing song for baby - Brahms wrote a good one" , "Soporific song" , "Song to send a child asleep" , "Soothing song for infants" , "composition" .)