Horses stood nervously in group (10)
I believe the answer is:
bloodstock
'horses' is the definition.
(term for horses considered collectively)
'stood nervously in group' is the wordplay.
'nervously' is an anagram indicator.
'in' indicates putting letters inside.
'group' becomes 'block' (a group or cluster of things).
'stood' anagrammed gives 'odsto'.
'odsto' placed within 'block' is 'BLOODSTOCK'.
(Other definitions for bloodstock that I've seen before include "pedigree horses" , "Thoroughbreds" , "Stable supply source" .)