Specialist on Underground, extremely pleased with herself (5)
I believe the answer is:
proud
'pleased with herself' is the definition.
'specialist on underground extremely' is the wordplay.
'specialist' becomes 'pro'.
'on' means one lot of letters go next to another.
'extremely' suggests removing the centre.
'underground' with its middle removed is 'ud'.
'pro'+'ud'='PROUD'
(Other definitions for proud that I've seen before include "Conscious of one's own dignity" , "Arrogant - sticking out" , "Guilty of deadly sin" , "With high self-opinion" , "Haughty - sticking out" .)