It's clear son is hiding in den (4)
I believe the answer is:
nest
'den' is the definition.
(I've seen this before)
'it's clear son is hiding in' is the wordplay.
'it's clear' becomes 'net' (as in net profit).
'son' becomes 's'.
'is hiding in' is an insertion indicator.
'net' going around 's' is 'NEST'.
(Other definitions for nest that I've seen before include "Shelter in which to lay eggs" , "Snug retreat" , "Ingredient of Chinese soup" , "Bird's house" , "Home to a bird" .)