They're of service in getting horses out (7)
I believe the answer is:
marines
'they're of service' is the definition.
The definition suggests an adverb but the answer is not.
'in getting horses out' is the wordplay.
I cannot quite see how this works, but
'in' is present in the answer.
'horses' could be 'mares' (female horses) and 'mares' is present in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for marines that I've seen before include "Soldiers" , "elite corps" , "Seminar for American troops" , "Proverbial recipients of tall stories" , "forces" .)