To moisten, it has to be water (5)
I believe the answer is:
bedew
'to moisten' is the definition.
(I've seen this in another clue)
'be water' is the wordplay.
'water' becomes 'dew' (dew is a kind of water).
'be'+'dew'='BEDEW'
'it has to' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for bedew that I've seen before include "Sprinkle with water (poetic)" , "Moisten (poetic)" .)