It runs into the sea (4)
I believe the answer is:
pier
'the sea' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'it runs' is the wordplay.
'it' becomes 'pie' (I can't justify this - if you can you should believe this answer much more).
'runs' becomes 'r' (cricket abbreviation).
'pie'+'r'='PIER'
'into' is the link.
Can you help me to learn more?
(Other definitions for pier that I've seen before include "seaside building" , "Supporting structure" , "seaside location" , "sight at the seaside" , "Support member" .)