Mary could be mistaken for being tipsy (5)
I believe the answer is:
merry
'being tipsy' is the definition.
(I've seen this before)
'mary could be mistaken' is the wordplay.
I cannot really understand how this works, but
'be mistaken' could be 'err' (I've seen this before) and 'err' is present in the answer.
The remaining letters 'my' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'for' is the link.
Can you help me to learn more?
(Other definitions for merry that I've seen before include "Convivial" , "Old King Cole was a ..... old soul" , "Drunk - happy" , "Joyous; tipsy (colloq.)" , "Mistresses Page and Ford of Windsor" .)