Sensible boy, English, engaged in study (8)
I believe the answer is:
reasoned
'sensible' is the definition.
(thesaurus)
'boy english engaged in study' is the wordplay.
'boy' becomes 'son' (synonyms).
'english' becomes 'e' (abbreviation).
'engaged in' is an insertion indicator.
'study' becomes 'read' (synonyms).
'son'+'e'='sone'
'sone' inserted into 'read' is 'REASONED'.
(Other definitions for reasoned that I've seen before include "Well thought out" , "Argued logically" , "made a logical case" , "Deduced or concluded" , "Logically valid" .)