Complete the document providing I am unable to be serious (11)
I believe the answer is:
significant
'serious' is the definition.
(thesaurus)
'complete the document providing i am unable' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'providing' could be 'if' (eg 'providing it's sunny' means 'if it's sunny') and 'if' is present in the answer.
'i' could be 'a' and 'a' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'to be' is the link.
Can you help me to learn more?
(Other definitions for significant that I've seen before include "Indicative of something" , "Having meaning" , "Material" , "pregnant" , "Consequential" .)