The French and Irish in den (4)
I believe the answer is:
lair
'den' is the definition.
(synonyms)
'the french and irish' is the wordplay.
'the french' becomes 'la' ('the' in French).
'and' is a charade indicator (letters next to each other).
'irish' becomes 'IR' (abbreviation).
'la'+'ir'='LAIR'
'in' is the link.
(Other definitions for lair that I've seen before include "mire where he lives" , "Den of a wild animal" , "Home of a beast" , "Earth" , "Supposed large ape, the Bigfoot" .)