Excessive burns doused periodically (5)
I believe the answer is:
undue
'excessive' is the definition.
('undue' can be a synonym of 'excessive')
'burns doused periodically' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'burns'+'doused'='burnsdoused'
The alternate letters of 'burnsdoused' are 'UNDUE'.
(Other definitions for undue that I've seen before include "Excessive - unwarranted" , "Improper" , "Unwarranted, inordinate" , "More than appropriate" , "Excessive amount" .)