Son breaks mirror in recess (4)
I believe the answer is:
apse
'recess' is the definition.
(apse is a kind of recess)
'son breaks mirror' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'breaks' means one lot of letters goes inside another.
'mirror' becomes 'ape' (I've seen this in other clues).
's' put within 'ape' is 'APSE'.
'in' is the link.
(Other definitions for apse that I've seen before include "inside church?" , "Recess in church wall" , "Semicircular recess in church" , "Rounded end of building" , "space for altar" .)