After excellent exercise, son turns brown (5)
I believe the answer is:
sepia
'brown' is the definition.
(I know that sepia is a type of brown)
'after excellent exercise son turns' is the wordplay.
'after' says to put letters next to each other.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'exercise' becomes 'pe' (Physical Education).
'son' becomes 's' (genealogical abbreviation for son).
'turns' shows that the letters should be reversed in order.
'ai'+'pe'='aipe'
'aipe'+'s' is 'aipes'.
'aipes' in reverse letter order is 'SEPIA'.
(Other definitions for sepia that I've seen before include "Shade of brown, as in old photographs" , "Rich reddish-brown colour" , "Reddish-brown colour as in old photos" , "cuttle" , "Brown of old photos" .)