The water was running when I came in (4)
I believe the answer is:
rain
'the water' is the definition.
(rain is a kind of water)
'i came in' is the wordplay.
'came' becomes 'ran' (running is a kind of coming).
'in' is an insertion indicator.
'i' placed into 'ran' is 'RAIN'.
'was running when' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for rain that I've seen before include "Storm" , "Precipitation" , "See 12" , "bad weather" , "Weather - shower" .)