Length of previous pavilion (6)
I believe the answer is:
extent
'length' is the definition.
(I've seen this before)
'previous pavilion' is the wordplay.
'previous' becomes 'ex' (prefix meaning former or onetime).
'pavilion' becomes 'tent' (I've seen this in another clue).
'ex'+'tent'='EXTENT'
'of' acts as a link.
(Other definitions for extent that I've seen before include "Length" , "Area; size" , "Magnitude - scope" , "Area covered by something" , "stretch" .)