You said first gent left if you start to make it unpleasant to look at (6)
I believe the answer is:
uglify
'make it unpleasant to look at' is the definition.
I can't tell whether this defines the answer.
'you said first gent left if you start' is the wordplay.
'you said' becomes 'U' (homophone of 'you').
'first gent' becomes 'g' (1st letter of 'gent').
'left' becomes 'l' (common abbreviation).
'you start' becomes 'y' (I've seen this in other clues).
'u'+'g'+'l'+'if'+'y'='UGLIFY'
'to' acts as a link.
Can you help me to learn more?
(Another definition for uglify that I've seen is " Make repulsive in appearance".)