Starting with Virgo, Zodiac sign alters (6)
I believe the answer is:
varies
'alters' is the definition.
('vary' can be a synonym of 'alter')
'starting with virgo zodiac sign' is the wordplay.
'starting with' indicates taking the first letters.
'zodiac sign' becomes 'aries' (I've seen this before).
The initial letter of 'virgo' is 'v'.
'v'+'aries'='VARIES'
(Other definitions for varies that I've seen before include "Makes or becomes different, changes" , "Changes, deviates" , "Differs, changes" , "Tends to change" , "'Alters, changes (6)'" .)