Instant cure initially found in most of samples (5)
I believe the answer is:
trice
'instant' is the definition.
('trice' can be a synonym of 'instant')
'cure initially found in most of samples' is the wordplay.
'initially' indicates taking the first letters.
'found in' is an insertion indicator.
'most of' means to remove the last letter.
'samples' becomes 'tries' ('try' can be a synonym of 'sample').
The initial letter of 'cure' is 'c'.
'tries' with its last letter removed is 'trie'.
'c' inserted into 'trie' is 'TRICE'.
(Other definitions for trice that I've seen before include "Next to no time" , "Haul" , "Instant of time" , "Short time" , "Old use for three times" .)