Being still not out, a trier endlessly bats (7)
I believe the answer is:
inertia
'being still' is the definition.
(inertia is stillness or lack of movement)
'not out a trier endlessly bats' is the wordplay.
'not out' becomes 'in' ('in' is the opposite to 'out').
'endlessly' means to remove the last letter.
'bats' indicates anagramming the letters (slang for mad).
'a'+'trier'='atrier'
'atrier' with its last letter taken off is 'atrie'.
'atrie' with letters rearranged gives 'ertia'.
'in'+'ertia'='INERTIA'
(Other definitions for inertia that I've seen before include "'Sluggishness, indolence (7)'" , "I retain, being without energy" , "Apathy" , "inclination not to budge" , "Passivity" .)