Something in the blood that gets moorhen moving (7)
I believe the answer is:
hormone
'something in the blood' is the definition.
(blood contains hormones)
'moorhen moving' is the wordplay.
'moving' indicates an anagram.
'moorhen' with letters rearranged gives 'HORMONE'.
'that gets' is the link.
(Other definitions for hormone that I've seen before include "therapy to replace this?" , "Glandular secretion with stimulating properties" , "something stimulating" , "Secretion affecting behaviour" , "Secretion from endocrine gland" .)