Increase production of gunmetal endlessly (7)
I believe the answer is:
augment
'increase' is the definition.
(augmenting is a kind of increasing)
'production of gunmetal endlessly' is the wordplay.
'production of' is an anagram indicator.
'endlessly' means to remove the last letter.
'gunmetal' with its final letter taken away is 'gunmeta'.
'gunmeta' is an anagram of 'AUGMENT'.
(Other definitions for augment that I've seen before include "Make greater in size, number, etc." , "Make larger" , "get longer" , "Expand" , "Swell" .)