Became unfriendly, so didn't drop in (4,3)

'became unfriendly so didn't' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'drop in' is the wordplay.
I cannot quite understand how this works, but
'drop' could be 'fell' (synonyms) and 'fell' is found in the answer.
The remaining letters 'out' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?