Determination to secure small delivery to mill? (5)
I believe the answer is:
grist
'delivery to mill?' is the definition.
(as in the phrase grist to the mill)
'determination to secure small' is the wordplay.
'determination' becomes 'grit' (grit can mean determination or tenacity).
'to secure' is an insertion indicator (inserted letters are secured inside).
'small' becomes 's' (abbreviation - e.g. clothes size).
'grit' placed around 's' is 'GRIST'.
(Other definitions for grist that I've seen before include "Ground grain" , "may be put through the mill" , "Corn to be ground" , "Grain for the mill" , "what miller needs" .)