Make beer of calibre - weak (4)
I believe the answer is:
brew
'make beer' is the definition.
(I've seen this in another clue)
'of calibre weak' is the wordplay.
'of' indicates the answer is hidden within the clue.
'BREW' is hidden in the letters of 'calibre weak'.
(Other definitions for brew that I've seen before include "Make beer by boiling and fermentation" , "Make, eg, beer" , "Make beer; drink of tea" , "Ferment rather than distil" , "Infusion" .)