... a source of inspiration - regularly see reaction (5)
I believe the answer is:
erato
'a source of inspiration' is the definition.
(one of the muses in Greek myth)
'regularly see reaction' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'see'+'reaction'='seereaction'
The alternate letters of 'seereaction' are 'ERATO'.
(Other definitions for erato that I've seen before include "inspiration for poetry" , "Arts patron" , "Who may have inspired poetry" , "Poetic Muse ; eighteen (anag.)" , "Clio's sister" .)