Increase when a relation, Meg, wanders in (7)
I believe the answer is:
augment
'increase' is the definition.
(I know that augment is a more specific form of the action increase)
'a relation meg wanders in' is the wordplay.
'a relation' becomes 'aunt' (aunt is a kind of relation).
'wanders' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'meg' anagrammed gives 'gme'.
'aunt' going around 'gme' is 'AUGMENT'.
'when' acts as a link.
(Other definitions for augment that I've seen before include "Add to, increase" , "Expand" , "Supplement" , "Grow - intensify" , "boost" .)