Is impressed by better jam (9)
I believe the answer is:
improvise
'jam' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how one could define the other.
'is impressed by better' is the wordplay.
'impressed by' indicates putting letters inside.
'better' becomes 'improve' (synonyms).
'is' placed into 'improve' is 'IMPROVISE'.
Can you help me to learn more?
(Other definitions for improvise that I've seen before include "Ad-lib" , "wing it" , "Talk without notes" , "Create without preparation" , "Perform without preparation" .)