Lake with small peninsula is wider than it is deep (9)
I believe the answer is:
landscape
'wider than it is deep' is the definition.
I can't tell whether this defines the answer.
'lake with small peninsula' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'with' becomes 'and'.
'small peninsula' becomes 'scape' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'l'+'and'+'scape'='LANDSCAPE'
'is' is the link.
Can you help me to learn more?
(Other definitions for landscape that I've seen before include "Piece of inland scenery" , "Countryside for a painter" , "Artwork" , "Oil perhaps" , "improve grounds" .)