Believe fit to be great (12)
I believe the answer is:
considerable
'great' is the definition.
(I've seen this before)
'believe fit' is the wordplay.
'believe' becomes 'consider' (considering is a kind of believing).
'fit' becomes 'able' ('able' can be similar in meaning to 'fit'**).
'consider'+'able'='CONSIDERABLE'
'to be' is the link.
(Other definitions for considerable that I've seen before include "Significant" , "Major" , "Fairly large or numerous" , "Great" , "worth entertaining?" .)