Within a couple of days, the fruit became mottled (7)
I believe the answer is:
dappled
'mottled' is the definition.
('dapple' can be a synonym of 'mottle')
'within a couple of days the fruit' is the wordplay.
'within' indicates putting letters inside.
'a couple of' shows that letters should be duplicated.
'days' becomes 'd'.
'the fruit' becomes 'apple' (apple is a kind of fruit).
'd' doubled is 'dd'.
'dd' going around 'apple' is 'DAPPLED'.
'became' acts as a link.
(Other definitions for dappled that I've seen before include "spotted with colour" , "Spotty" , "Mottled" , "Marked with spots or patches or shade" .)