Long, long ship deserted on river (7)
I believe the answer is:
spondee
'long long' is the definition.
(poetic foot of two long syllables)
'ship deserted on river' is the wordplay.
'deserted' means to remove the middle letters.
'river' becomes 'dee' (River Dee).
'ship' with its middle taken out is 'sp'.
'sp'+'on'+'dee'='SPONDEE'
(Other definitions for spondee that I've seen before include "No speed required for poetic metrical foot" , "bit of verse" , "Metric foot of two long syllables" .)