It follows the herd up street, first of all (4)
I believe the answer is:
thus
'it follows' is the definition.
(both can mean 'therefore')
'the herd up street first of all' is the wordplay.
'first of all' indicates taking the first letters.
The initial letters of 'the herd up street' is 'THUS'.
(Other definitions for thus that I've seen before include "As a result" , "In this way" , "Shut (anag.)" , "'So, therefore (4)'" , "In this manner" .)