Diarmuid, Irishman holding back what's in platinum alloy? (7)
I believe the answer is:
iridium
'what's in platinum alloy?' is the definition.
(as in platinum-iridium alloy)
'diarmuid irishman holding back' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'back' says the letters should be written in reverse.
'muidiri' is hidden within 'diarmuid irishman'.
'muidiri' back-to-front is 'IRIDIUM'.
(Other definitions for iridium that I've seen before include "Corrosion inhibitor" , "Metallic element resistant to corrosion, atomic no. 77" , "Heavy brittle metal used in alloys, Ir" , "Hard metal element, Ir" , "Silvery-white metallic element" .)