Dearest son is inside cupboard (7)
I believe the answer is:
closest
'dearest' is the definition.
('close' can be similar in meaning to 'dear')
'son is inside cupboard' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'is inside' indicates putting letters inside.
'cupboard' becomes 'closet' (type of cupboard).
's' inserted into 'closet' is 'CLOSEST'.
(Other definitions for closest that I've seen before include "Superlatively dense" , "Nearest or meanest" , "Most secretive" , "Most intimate" , "Nearest or most sultry" .)