Bird fed the tiny worms outside, you wrongly assumed? (8,5)
I believe the answer is:
identity theft
'bird fed' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'the tiny worms outside you wrongly assumed?' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
an anagram of 'tiny' is 'nity' which is located in the remaining letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for identity theft that I've seen before include "Form of fraud" , "A crime of personal fraud" , "Type of fraud" , "fraudulent practice" .)