Bottomless river: source in Pennines (4)
I believe the answer is:
deep
'bottomless' is the definition.
(similar in meaning)
'river source in pennines' is the wordplay.
'river' becomes 'dee' (River Dee).
'source in' says to take the initial letters.
The first letter of 'pennines' is 'p'.
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "Cavernous" , "Extending far down" , "dangerous for swimming?" , "Main" , "Profound - engrossed" .)