It's clear son is hiding in den (4)
I believe the answer is:
Here is my best explanation:
'den' is the definition.
(I've seen this before)
'it's clear son is hiding in' is the subsidiary indication.
'it's clear' becomes 'net' (as in net profit).
'son' becomes 's'.
'is hiding in' is an insertion indicator.
'net' enclosing 's' is 'nest'.
(Other definitions for nest that I've seen before include "Snuggle in like bird", "Retreat!", "stack", "Home to a bird", "Bird's home", "Structure to lay eggs", "Lay eggs here", "Home for birds", "Roost" and "Stack of tables".)