One keeps it in reserve (8)
I believe the answer is:
distance
'reserve' is the definition.
(I've seen this before)
'one keeps it' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for distance that I've seen before include "Length between points - remoteness" , "What's kept in reserve" , "The gap between two places" , "Ascend it from one point to another" , "Full extent of fighting" .)