More confident underwriter drops in (5)
I believe the answer is:
surer
'more confident' is the definition.
('sure' can be similar in meaning to 'confident')
'underwriter drops in' is the wordplay.
'underwriter' becomes 'insurer' (I've seen this before).
'drops' indicates named letters should be taken away.
'insurer' with 'in' taken away is 'SURER'.
(Other definitions for surer that I've seen before include "More certain and positive" , "more right?" , "faster" .)