Sturdily built son hiding in copse (8)
I believe the answer is:
thickset
'sturdily built' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs occasionally mean the same thing.
'son hiding in copse' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'hiding in' indicates putting letters inside.
'copse' becomes 'thicket' (synonyms).
's' placed inside 'thicket' is 'THICKSET'.
Can you help me to learn more?
(Other definitions for thickset that I've seen before include "Concentrated" , "Heavily built" , "squat" , "positioned close together" , "Closely planted" .)