A foot of water in the diocese (7)
I believe the answer is:
spondee
'a foot' is the definition.
(spondee is a kind of foot)
'water in the diocese' is the wordplay.
'water' becomes 'pond' (pond is a kind of water).
'in the' means one lot of letters goes inside another.
'diocese' becomes 'see' (I've seen this before).
'pond' inserted inside 'see' is 'SPONDEE'.
'of' acts as a link.
(Other definitions for spondee that I've seen before include "No speed required for poetic metrical foot" , "Metric foot of two long syllables" , "bit of verse" .)