Ideology is mesmerising at first (3)
I believe the answer is:
ism
'ideology' is the definition.
(I've seen this before)
'is mesmerising at first' is the wordplay.
'at first' says to take the initial letters.
The initial letter of 'mesmerising' is 'm'.
'is'+'m'='ISM'
(Other definitions for ism that I've seen before include "form of belief?" , "Informally, any doctrine or practice" , "Ideological movement" , "Distinctive theory" , "specific doctrine" .)