Gear Len used to expand (7)
I believe the answer is:
enlarge
'to expand' is the definition.
(I've seen this before)
'gear len used' is the wordplay.
'used' indicates an anagram.
'gear'+'len'='gearlen'
'gearlen' anagrammed gives 'ENLARGE'.
(Other definitions for enlarge that I've seen before include "scale up" , "Grow" , "Amplify" , "Make bigger in general" , "Dilate" .)