In altered mode, boiler lamented (7)
I believe the answer is:
mourned
'lamented' is the definition.
(I've seen this in another clue)
'in altered mode boiler' is the wordplay.
'in' is an insertion indicator.
'altered' indicates an anagram.
'boiler' becomes 'urn'.
'mode' is an anagram of 'moed'.
'moed' going around 'urn' is 'MOURNED'.
(Other definitions for mourned that I've seen before include "No demur about the late departed" , "Grieved for, lamented" , "Grieved, sorrowed" .)