Accept heavenly body on Tom withers when neglected (4,2,5,4)
I believe the answer is:
come to terms with
'accept' is the definition.
(I've seen this before)
'heavenly body on tom withers when neglected' is the wordplay.
'heavenly body' becomes 'comet' (comet is a kind of heavenly body).
'on' is a charade indicator (letters next to each other).
'when neglected' indicates an anagram.
'tom'+'withers'='tomwithers'
'tomwithers' anagrammed gives 'otermswith'.
'comet'+'otermswith'='COME TO TERMS WITH'
(Another definition for come to terms with that I've seen is " Accept".)