Increase extent of victory over gutless Dundee (5)
I believe the answer is:
widen
'increase extent of' is the definition.
(to widen is to extend)
'victory over gutless dundee' is the wordplay.
'victory' becomes 'win' (I've seen this before).
'over' indicates putting letters inside.
'gutless' means to remove the middle letters.
'dundee' with its middle taken out is 'de'.
'win' placed around 'de' is 'WIDEN'.
(Other definitions for widen that I've seen before include "Broaden - enlarge" , "Expand" , "Make broader" , "Dilate" , "Stretch sideways" .)