Pee-pee or leak (4)
I believe the answer is:
weep
'leak' is the definition.
(I've seen this before)
'pee-pee' is the wordplay.
'pee' becomes 'wee' ('wee' can be a synonym of 'pee').
'pee' becomes 'p'.
'wee'+'p'='WEEP'
'or' is the link.
(Other definitions for weep that I've seen before include "Cry, shed tears" , "Seep slowly out" , "lament" , "Greet" , "pipe" .)