Animal wheeling to take exact position between two groups of soldiers retreating (7)
I believe the answer is:
rotifer
'animal wheeling' is the definition.
(type of microscopic called a wheel animal)
'to take exact position between two groups of soldiers retreating' is the wordplay.
'to take exact position' becomes 'fit' (to fit is to fill a position perfectly).
'between' means one lot of letters goes inside another.
'two groups of' becomes 're' (I can't explain this - if you can you should give a lot more credence to this answer).
'soldiers' becomes 'or' (Other Rank).
'retreating' shows that the letters should be reversed in order.
're'+'or'='reor'
'fit' inserted into 'reor' is 'refitor'.
'refitor' back-to-front is 'ROTIFER'.
(Other definitions for rotifer that I've seen before include "Simple animal" , "Aquatic creature" , "Tiny animal" .)