She has fifty-one pounds, but a diamond would suit her! (3)
I believe the answer is:
lil
'she' is the definition.
'fifty-one pounds but a diamond would suit her' is the wordplay.
I cannot quite understand how this works, but
'fifty' could be 'li' (I've seen this in other clues) and 'li' is located in the answer.
'pounds' could be 'l' (currency sign) and 'l' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'has' acts as a link.
Can you help me to learn more?
(Another definition for lil that I've seen is " 'Diamond - - ', 17's Broadway hit".)