If not, a fund is diminished and diminished! (6)
I believe the answer is:
unless
'if not' is the definition.
(I've seen this in another clue)
'a fund is diminished and diminished' is the wordplay.
'a fund' becomes 'u' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'is diminished and' becomes 'n' (common abbreviation for 'and'; 'diminished' indicates a short form).
'diminished' becomes 'less' (I've seen this in another clue).
'u'+'n'+'less'='UNLESS'
Can you help me to learn more?
(Other definitions for unless that I've seen before include "Except, save" , "Though" , "bar" , "Except on the condition that" , "Conjunction meaning except when" .)