Regularly ignored fungal yeast's beginning to be unsightly (4)
I believe the answer is:
'be unsightly' is the definition.
(similar in meaning)
'regularly ignored fungal yeast's beginning' is the wordplay.
'regularly ignored' indicates alternate letters.
'beginning' suggests taking the first letters.
The initial letter of 'yeasts' is 'y'.
The alternate letters of 'fungal' are 'ugl'.
'to' acts as a link.
(Other definitions for ugly that I've seen before include "Difficult to look at" , "Displeasing to the senses" , "Unpleasant to look at" , "Awful" , "Nasty to look at" .)