Allow one day to gather one hundred plants (8)
I believe the answer is:
lettuces
'plants' is the definition.
(lettuce is a kind of plant)
'allow one day to gather one hundred' is the wordplay.
'allow' becomes 'let' ('let' can be a synonym of 'allow').
'one day' becomes 'tues' (abbreviation for Tuesday**).
'to gather' is an insertion indicator.
'one hundred' becomes 'c' (Roman numeral).
'tues' placed around 'c' is 'tuces'.
'let'+'tuces'='LETTUCES'
(Other definitions for lettuces that I've seen before include "Cos and Iceberg, say" , "Green leaves for salad" , "Green leaves for salads" .)