Weak theory oddly incorporated in system (6)
I believe the answer is:
watery
'weak' is the definition.
(I know this)
'theory oddly incorporated in system' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'incorporated in' indicates putting letters inside.
'system' becomes 'way' (I can't explain this - if you can you should believe this answer much more).
The alternate letters of 'theory' are 'ter'.
'ter' placed into 'way' is 'WATERY'.
Can you help me to learn more?
(Other definitions for watery that I've seen before include "Overly diluted, thin" , "Weak, too diluted" , "Insipid" , "(Of food) thin, tasteless" , "Dilute; weak, pale" .)