To save is difficult with nothing coming in (5)
I believe the answer is:
hoard
'to save' is the definition.
(hoard can mean to save or keep)
'difficult with nothing coming in' is the wordplay.
'difficult' becomes 'hard' (synonyms).
'with nothing' becomes 'o' (I can't explain this - if you can you should give a lot more credence to this answer).
'coming in' is an insertion indicator.
'hard' placed around 'o' is 'HOARD'.
'is' is the link.
Can you help me to learn more?
(Other definitions for hoard that I've seen before include "Squirrel away" , "Secret stash" , "keep in reserve" , "Accumulated store" , "Accumulate" .)