Minor adjustment lacking ability to support argument's conclusion (5)
I believe the answer is:
tweak
'minor adjustment' is the definition.
(I've seen this before)
'lacking ability to support argument's conclusion' is the wordplay.
'lacking ability' becomes 'weak' (I can't justify this - if you can you should believe this answer much more).
'to support' means one lot of letters go next to another (in a down clue, some letters hold up others).
'conclusion' indicates one should take the final letters.
The final letter of 'argument' is 't'.
'weak' after 't' is 'TWEAK'.
Can you help me to learn more?
(Other definitions for tweak that I've seen before include "Fine-tune, twist" , "Make small adjustment" , "Fine adjustment (colloq.)" , "[TUG]" , "Edit" .)