Bottomless river with source in Pennines (4)
I believe the answer is:
deep
'bottomless' is the definition.
(similar in meaning)
'river with source in pennines' is the wordplay.
'river' becomes 'dee' (River Dee).
'with' means one lot of letters go next to another.
'source in' says to take the initial letters.
The initial letter of 'pennines' is 'p'.
'dee'+'p'='DEEP'
(Other definitions for deep that I've seen before include "Profound - engrossed" , "hard to understand" , "dangerous for swimming?" , "Profound - cunning - sea - bass" , "Extending far down" .)