Might of monster clad in iron (5)
I believe the answer is:
force
'might' is the definition.
(I've seen this in another clue)
'monster clad in iron' is the wordplay.
'monster' becomes 'orc' (fantasy creature).
'clad in' means one lot of letters goes inside another.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'orc' put within 'fe' is 'FORCE'.
'of' acts as a link.
(Other definitions for force that I've seen before include "Northern waterfall" , "Impetus" , "Powerful effect or influence" , "fall" , "Strength - compulsion" .)