Very little pressure to turn on the waterworks (4)
I believe the answer is:
weep
'turn on the waterworks' is the definition.
(I've seen this before)
'very little pressure' is the wordplay.
'very little' becomes 'wee'.
'pressure' becomes 'p' (abbreviation).
'wee'+'p'='WEEP'
'to' is the link.
(Other definitions for weep that I've seen before include "Express grief or sorrow" , "Turn on the waterworks" , "leak" , "Bawl" , "Display sorrow" .)