Former PM losing head in retreat (4)
I believe the answer is:
lair
'retreat' is the definition.
(both can mean a private hideout)
'former pm losing head' is the wordplay.
'former pm' becomes 'Blair' (prime minister Tony Blair).
'losing head' means to remove the first letter.
'blair' with its first letter taken off is 'LAIR'.
'in' is the link.
(Other definitions for lair that I've seen before include "Hiding place, den" , "office?" , "Habitation of a wild animal" , "Home of a beast" , "enclosure" .)