Sign is inconsistent? Not very (5)
I believe the answer is:
aries
'sign' is the definition.
(star sign)
'is inconsistent? not very' is the wordplay.
'is inconsistent?' becomes 'varies' (something that varies is inconsistent).
'not' indicates named letters should be taken away.
'very' becomes 'v' (common abbreviation).
'varies' with 'v' removed is 'ARIES'.
(Other definitions for aries that I've seen before include "Ram star sign" , "House" , "Sheep" , "Stars" , "Zodiac ram" .)