Marrowfat pea resisted at first by lightweight woman (9)
I believe the answer is:
'marrowfat pea' is the definition.
(type of pea)
'resisted at first by lightweight woman' is the wordplay.
'at first' indicates taking the first letters.
'by lightweight' becomes 'ounce' (I've seen this before. I am not sure about the 'by' bit.).
'woman' becomes 'Val' (val is a name for a woman).
The initial letter of 'resisted' is 'r'.