Easy opportunity to bag stray reptile (8)
I believe the answer is:
terrapin
'reptile' is the definition.
(terrapin is a kind of reptile)
'easy opportunity to bag stray' is the wordplay.
'easy opportunity' becomes 'tapin' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to bag' means one lot of letters goes inside another.
'stray' becomes 'err' (to err is to stray or go wrong).
'tapin' going around 'err' is 'TERRAPIN'.
Can you help me to learn more?
(Other definitions for terrapin that I've seen before include "Little turtle" , "Small freshwater turtle" , "Partner I found for freshwater tortoise" , "Hardback" , "freshwater creature" .)