Agile son is given prize (4)
I believe the answer is:
spry
'agile' is the definition.
(synonyms)
'son is given prize' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'is given' means one lot of letters go next to another.
'prize' becomes 'pry' (pry can mean to prize or prise open).
's'+'pry'='SPRY'
(Other definitions for spry that I've seen before include "Sprightly" , "'Agile, nimble (4)'" , "Moving quickly and lightly" , "(Of an old person) lively" , "Nimble, brisk" .)