For the under eleven, diminish (7)
I believe the answer is:
subside
'diminish' is the definition.
(I know that subside is a type of weaken)
'for the under eleven' is the wordplay.
I cannot really see how this works, but
'under' could be 'sub' (I've seen this before) and 'sub' is found in the answer.
'eleven' could be 'side' (a football side) and 'side' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for subside that I've seen before include "Abate" , "Diminish" , "Sink, settle" , "Wane" , "Cave in" .)