Half have become fit — squash perhaps? (9)
I believe the answer is:
vegetable
'squash perhaps?' is the definition.
(squash is a kind of vegetable)
'half have become fit' is the wordplay.
'half' indicates taking half.
'become' becomes 'get' ('get' can be a synonym of 'become').
'fit' becomes 'able' ('able' can be similar in meaning to 'fit'**).
'have' with half the letters taken is 've'.
've'+'get'+'able'='VEGETABLE'
(Other definitions for vegetable that I've seen before include "Edible seed, root, stem, leaf or bulb" , "Plant, especially if edible" , "Such as 11 ac" , "Broccoli eg" , "Edible root or leaf, say" .)