What water may do at the pithead, you understand (4)
I believe the answer is:
seep
'what water' is the definition.
I can't judge whether this definition defines the answer.
'do at the pithead you understand' is the wordplay.
I cannot quite understand how this works, but
'pithead' could be 'p' (head letter of 'pit') and 'p' is found in the answer.
'understand' could be 'see' (synonyms) and 'see' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'may' acts as a link.
Can you help me to learn more?
(Other definitions for seep that I've seen before include "Come out slowly" , "Of liquid, ooze slowly out" , "Leak slowly, ooze" , "Trickle through slowly" , "dribble away" .)