Reserve cast put by lake (7)
I believe the answer is:
shyness
'reserve' is the definition.
(I've seen this in another clue)
'cast put by lake' is the wordplay.
'cast' becomes 'shy' (both can mean to throw or fling).
'put' says to put letters next to each other.
'by lake' becomes 'ness' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'shy'+'ness'='SHYNESS'
Can you help me to learn more?
(Other definitions for shyness that I've seen before include "lack of confidence" , "Retiring quality" , "Nervousness, timidity" , "Timidity in company" , "Bashfulness" .)