Stresses when grammar forms increase (6,2)
I believe the answer is:
tenses up
'stresses' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot see how one could define the other.
'grammar forms increase' is the wordplay.
'grammar forms' becomes 'tenses' (I can't explain this - if you can you should believe this answer much more).
'increase' becomes 'up' (upping is a kind of increasing).
'tenses'+'up'='TENSES UP'
'when' acts as a link.
Can you help me to learn more?
(Another definition for tenses up that I've seen is " becomes nervous".)