Regularly fierce, showing strong emotion (3)
I believe the answer is:
ire
'strong emotion' is the definition.
(I've seen this before)
'regularly fierce' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'fierce' are 'IRE'.
'showing' is the link.
(Other definitions for ire that I've seen before include "'Anger, wrath (3)'" , "Paddy" , "Losing head" , "exasperation" , "passionate feeling" .)