Dense group of trees in which little son hides (8)
I believe the answer is:
thickset
'dense group of trees in which little son hides' is the definition.
'thickset' can be an answer for 'dense' (densely or thickly arranged). I am not sure about the remainder of the definition.
'dense group of trees in which little son' is the wordplay.
'dense group of trees' becomes 'thicket' (I've seen this before).
'in which' indicates putting letters inside.
'little son' becomes 's' (abbreviation for son).
'thicket' placed around 's' is 'THICKSET'.
(Other definitions for thickset that I've seen before include "Concentrated" , "Solidly-built" , "sturdy" , "Brawny" , "positioned close together" .)