First wise men on retreat study 'thought' (8)
I believe the answer is:
imagined
'thought' is the definition.
('imagine' can be a synonym of 'think')
'first wise men on retreat study' is the wordplay.
'first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'wise men' becomes 'magi' (the three wise men from the nativity).
'on retreat' says the letters should be written backwards (to retreat is to go back).
'study' becomes 'den' (term for a study or office).
'den' in reverse letter order is 'ned'.
'i'+'magi'+'ned'='IMAGINED'
(Other definitions for imagined that I've seen before include "Supposed" , "Envisaged" , "thought" , "Dreamt up" , "Conceived of" .)