Bed's base son breaks (5)
I believe the answer is:
'bed's' is the definition.
(I've seen this before)
'base son breaks' is the wordplay.
'base' becomes 'root' (synonyms).
'son' becomes 's' (genealogical abbreviation for son).
'breaks' means one lot of letters goes inside another (inserted letters break into the word).
'root' placed around 's' is 'ROOST'.
(Other definitions for roost that I've seen before include "(Bird) settle for sleep" , "residence" , "suitable sleeping-place" , "Hen house; perch" , "Colony of bats; perch" .)