Campaigner badly scarred across middle of thumb (8)
I believe the answer is:
crusader
'campaigner' is the definition.
(I've seen this before)
'badly scarred across middle of thumb' is the wordplay.
'badly' indicates anagramming the letters.
'across' means one lot of letters goes inside another.
'middle of' says to take the centre.
The middle letter of 'thumb' is 'u'.
'scarred' is an anagram of 'crsader'.
'crsader' going around 'u' is 'CRUSADER'.
(Other definitions for crusader that I've seen before include "Champion" , "One campaigning vigorously" , "Richard I, for instance" , "Advocate for a cause - rude scar (anag)" , "Medieval expeditionary" .)