If out by £50, avoid any deficit (4)
I believe the answer is:
fill
'avoid any deficit' is the definition.
I can't judge whether this defines the answer.
'if out by ps50' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'by' means one lot of letters go next to another.
'ps50' becomes 'll' (I can't explain this - if you can you should believe this answer much more).
'if' with letters rearranged gives 'fi'.
'fi'+'ll'='FILL'
Can you help me to learn more?
(Other definitions for fill that I've seen before include "Appoint person to (post)" , "Completely occupy" , "Be suitable - . . . . the bill" , "Become satiated" , "Plug (a gap)" .)