Robert covers miles, nevertheless, finally fails (5)
I believe the answer is:
bombs
'fails' is the definition.
('bomb' can be a synonym of 'fail')
'robert covers miles nevertheless finally' is the wordplay.
'robert' becomes 'bob' (I've seen this before).
'covers' means one lot of letters goes inside another.
'miles' becomes 'm' (common abbreviation).
'finally' says to take the final letters.
The last letter of 'nevertheless' is 's'.
'bob' placed around 'm' is 'bomb'.
'bomb'+'s'='BOMBS'
(Other definitions for bombs that I've seen before include "Explosive devices" , "Fails horribly" , "Shells" , "Weapons" .)