Yearn to show a bit of panache (4)
I believe the answer is:
ache
'yearn' is the definition.
('ache' can be a synonym of 'yearn')
'a bit of panache' is the wordplay.
'bit of panache' becomes 'che' (I can't explain this - if you can you should believe this answer much more).
'a'+'che'='ACHE'
'to show' acts as a link.
Can you help me to learn more?
(Other definitions for ache that I've seen before include "Dull continuous pain" , "A dull lingering pain" , "burn" , "yearning" , "Feel sore" .)