Reduce each damaged pen (7)
I believe the answer is:
cheapen
'reduce' is the definition.
(I've seen this in another clue)
'each damaged pen' is the wordplay.
'damaged' is an anagram indicator.
'each' anagrammed gives 'chea'.
'chea'+'pen'='CHEAPEN'
(Other definitions for cheapen that I've seen before include "Degrade (something)" , "Decry or devalue" , "lower" , "Depreciate" , "Tried" .)