Increase is relative when false gem is inset (7)
I believe the answer is:
augment
'increase' is the definition.
(augmenting is a kind of increasing)
'relative when false gem is inset' is the wordplay.
'relative when' becomes 'aunt' (I can't justify this - if you can you should believe this answer much more).
'false' indicates anagramming the letters.
'is inset' indicates putting letters inside (some letters are set inside others).
'gem' is an anagram of 'gme'.
'aunt' placed around 'gme' is 'AUGMENT'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for augment that I've seen before include "Supplement" , "get longer" , "Grow - intensify" , "Expand" , "Enlarge or increase" .)