Hot iron beginning to ruin fissure (7)
I believe the answer is:
fervent
'hot' is the definition.
(synonyms)
'iron beginning to ruin fissure' is the wordplay.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'beginning to' indicates taking the first letters.
'fissure' becomes 'vent' (vent is a kind of fissure).
The initial letter of 'ruin' is 'r'.
'fe'+'r'+'vent'='FERVENT'
(Other definitions for fervent that I've seen before include "Intensely passionate" , "Zealous" , "Hot" , "Impassioned" , "Ardent" .)