Retrievers round up 1000 sheep (5)
I believe the answer is:
lambs
'sheep' is the definition.
(I have seen 'Young sheep ' mean 'lambs' so perhaps 'sheep' could also mean 'lambs')
'retrievers round up 1000' is the wordplay.
I cannot really understand how this works, but
'1000' could be 'm' (synonyms) and 'm' is found within the answer.
The remaining letters 'labs' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lambs that I've seen before include "See 17" , "Young sheep" .)