Exercises by boy capsized old boat (6)
I believe the answer is:
pedalo
'boat' is the definition.
(I know that pedalo is a type of boat)
'exercises by boy capsized old' is the wordplay.
'exercises' becomes 'pe' (Physical Education class).
'by' says to put letters next to each other.
'boy' becomes 'lad' (informal term for boy).
'capsized' is a reversal indicator (in a down clue, the word turns upside down).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'lad' written backwards gives 'dal'.
'pe'+'dal'+'o'='PEDALO'
(Other definitions for pedalo that I've seen before include "Foot-operated boat" , "Type of pleasure boat" , "Boat driven by foot power" , "Man-powered leisure boat" , "Foot-powered pleasure craft" .)