He was reportedly tight as he made up plot! (5)
I believe the answer is:
oates
'he made up plot' is the definition.
(Titus Oates fabricated the Popish Plot)
'he was reportedly tight' is the wordplay.
I cannot really understand how this works, but
'reportedly' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
The remaining letters 'oats' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'as' is the link.
(Other definitions for oates that I've seen before include "man who achieved 1 ac [POLE POSITION]" , "Very gallant gentleman" , "sacrifice" , "Member of Scott's expedition - "Popish Plot" conspirator" , "Self-sacrificing member of Scott's Antarctic team" .)