Sample initially put into sterile bag (5)
I believe the answer is:
purse
'bag' is the definition.
(I've seen this before)
'sample initially put into sterile' is the wordplay.
'initially' indicates taking the first letters.
'put into' indicates putting letters inside.
'sterile' becomes 'pure'.
The initial letter of 'sample' is 's'.
's' going within 'pure' is 'PURSE'.
(Other definitions for purse that I've seen before include "Sum of money as a prize - in pouch?" , "Knit" , "Contract orally" , "Money - contract" , "Prize funds" .)