Cross-country runner in army service (4)
I believe the answer is:
yser
'cross-country runner' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'in army service' is the wordplay.
'in' indicates the answer is hidden within the clue.
'YSER' is hidden in the letters of 'army service'.
Can you help me to learn more?
(Other definitions for yser that I've seen before include "European river" , "River rising in France" , "Glum" , "Belgian river associated with Flanders battleground" , "European flower" .)