Iron, not completely rigid, showing decay (6)
I believe the answer is:
fester
'decay' is the definition.
(I've seen this before)
'iron not completely rigid' is the wordplay.
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'not completely' means to remove the last letter.
'rigid' becomes 'stern' (I've seen this before).
'stern' with its final letter taken off is 'ster'.
'fe'+'ster'='FESTER'
'showing' is the link.
(Other definitions for fester that I've seen before include "Go septic or cause resentment" , "Decay" , "Rankle" , "Become rotten, bitter or irritated" , "Rot" .)