Underwear extremely visible? Bold! (5)
I believe the answer is:
brave
'bold' is the definition.
(similar in meaning)
'underwear extremely visible?' is the wordplay.
'underwear' becomes 'bra' (bra is a type of undergarment).
'extremely' suggests removing the centre (only the extremes of the word are used).
'visible' with its middle removed is 've'.
'bra'+'ve'='BRAVE'
(Other definitions for brave that I've seen before include "Heroic, valiant" , "Film about William Wallace" , "Showing courage" , "Courageous, dauntless" , "Egg-shaped" .)