Change a shirt not up to standard (5)
I believe the answer is:
tweak
'change' is the definition.
(I've seen this before)
'a shirt not up to standard' is the wordplay.
'a shirt' becomes 't' (as in a T-shirt).
'not up to standard' becomes 'weak' (I can't justify this - if you can you should give a lot more credence to this answer).
't'+'weak'='TWEAK'
Can you help me to learn more?
(Other definitions for tweak that I've seen before include "Fine-tune, twist" , "Pinch and twist" , "Make small adjustments" , "Make minor adjustments" , "Fine adjustment (colloq.)" .)