Fanatical Scotsman regularly wild (5)
I believe the answer is:
rabid
'fanatical' is the definition.
(synonyms)
'scotsman regularly wild' is the wordplay.
'scotsman' becomes 'Rab' (Rab is a Scottish name).
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'wild' are 'id'.
'rab'+'id'='RABID'
(Other definitions for rabid that I've seen before include "Affected by 24 across" , "Violent; hydrophobic" , "Extreme, fanatical" , "Extremely fanatical" , "Infected with hydrophobia" .)