Uniform beginning to narrow somewhat below the belt (8)
I believe the answer is:
unfairly
'below the belt' is the definition.
(below the belt means unfair)
'uniform beginning to narrow somewhat' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'beginning to' says to take the initial letters.
'somewhat' becomes 'fairly' (synonyms).
The first letter of 'narrow' is 'n'.
'u'+'n'+'fairly'='UNFAIRLY'
(Other definitions for unfairly that I've seen before include "Without justice" , "Inequitably, wrongly" , "Unjustly" , "dishonestly" .)