Around Reading, one's selling fish (5)
I believe the answer is:
trout
'fish' is the definition.
(trout is a kind of fish)
'around reading one's selling' is the wordplay.
'around' is an insertion indicator.
'reading' becomes 'r' (one of the traditional three R's of education).
'one's selling' becomes 'tout' (someone selling or touting things).
'r' placed within 'tout' is 'TROUT'.
(Other definitions for trout that I've seen before include "Tutor (anag.)" , "air" , "Irritating old person" , "Performance by five players" , "unpleasant old woman" .)