Locks are the key to its existence (5)
I believe the answer is:
canal
'locks are' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'the key to its existence' is the wordplay.
I cannot really understand how this works, but
'key' could be 'c' (I've seen this in other clues) and 'c' is found in the answer.
The remaining letters 'anal' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for canal that I've seen before include "Man-made watercourse" , "eg Grand Union" , "Artificial waterway as at Panama (5)" , "Grand Union perhaps" , "Long, narrow strip of water" .)