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 can't justify this - if you can you should give a lot more credence to this answer).
'to bag' is an insertion indicator.
'stray' becomes 'err' (to err is to stray or go wrong).
'tapin' placed 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" .)