Thinker needs this in middle of Cumbria beside waterfall (5)
I believe the answer is:
brain
'thinker' is the definition.
(I've seen this in another clue)
'this in middle of cumbria beside waterfall' is the wordplay.
'this in middle of' means to look at the middle letters (I've seen 'in middle' mean this).
'beside' says to put letters next to each other (I've seen this in other clues).
'waterfall' becomes 'rain' (rain is water falling down).
The middle letter of 'cumbria' is 'b'.
'b'+'rain'='BRAIN'
'needs' acts as a link.
(Other definitions for brain that I've seen before include "Encephalon" , "It controls the central nervous system" , "Thinking thing" , "Organ of the intellect" , "Haemorrhaging of talent" .)