Terrible actor was famously useless Lear first of all (5)
I believe the answer is:
awful
'terrible' is the definition.
('awful' can be a synonym of 'terrible')
'actor was famously useless lear first of all' is the wordplay.
'first of all' says to take the initial letters.
The first letters of 'actor was famously useless lear' is 'AWFUL'.
(Other definitions for awful that I've seen before include "Really terrible, deplorable" , "Horrible" , "Extremely bad or unpleasant" , "Exceptionally bad" , "gruesome!" .)