River given inner layer to be efficient (11)
I believe the answer is:
streamlined
'be efficient' is the definition.
(similar in meaning)
'river given inner layer' is the wordplay.
'river' becomes 'stream' (I've seen this in another clue).
'given inner layer' becomes 'lined' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'stream'+'lined'='STREAMLINED'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for streamlined that I've seen before include "Designed to reduce wind resistance" , "made more efficient" , "Shaped for speed" , "Designed for smooth flow" , "Of sleek appearance" .)