Tim's one order, to add water (7)
I believe the answer is:
moisten
'water' is the definition.
(I've seen this before)
'tim's one order' is the wordplay.
'order' indicates an anagram.
'tims'+'one'='timsone'
'timsone' is an anagram of 'MOISTEN'.
'to add' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for moisten that I've seen before include "Make damp or slightly wet" , "It's on me (anag)" , "Make damp, say lips with tongue" , "What one's eyes may do!" , "contrary to sixteen down [DEHYDRATE]!" .)