Flock is going to race madly round back of pen (12)
I believe the answer is:
congregation
'flock' is the definition.
(thesaurus)
'going to race madly round back of pen' is the wordplay.
'madly' is an anagram indicator.
'round' is an insertion indicator.
'back of' suggests the final letters.
The final letter of 'pen' is 'n'.
'going'+'to'+'race'='goingtorace'
'goingtorace' anagrammed gives 'cogregation'.
'cogregation' placed around 'n' is 'CONGREGATION'.
'is' is the link.
(Other definitions for congregation that I've seen before include "Group assembling for worship" , "Group of worshippers" .)