It's tough not replacing it in central fund (6)
I believe the answer is:
knotty
'it's tough' is the definition.
(I've seen this in another clue)
'not replacing it in central fund' is the wordplay.
I cannot quite understand how this works, but
'not' could be 'no' (negative) and 'no' is located in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for knotty that I've seen before include "Difficult - having hard lumps (of wood)" , "Very difficult - complex" , "Convoluted" , "Tricky" , "Extremely difficult" .)