Tom informally watched you (6)
I believe the answer is:
sawyer
'tom' is the definition.
(I've seen this before)
'watched you' is the wordplay.
'watched' becomes 'saw' (seeing is a kind of watching).
'you' becomes 'yer' ('yer' is a dialect form of 'you').
'saw'+'yer'='SAWYER'
'informally' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sawyer that I've seen before include "Mark Twain character, Tom --" , "Huckleberry Finn's friend Tom ___" , "Mark Twain's character, Tom" , "Would you say worker" , "working on deal?" .)