Bottom’s new appearance, involving donkey’s head (5)
I believe the answer is:
nadir
'bottom's' is the definition.
(I've seen this in another clue)
'new appearance involving donkey's head' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'appearance' becomes 'air' (both can mean an impression or character).
'involving' is an insertion indicator.
'head' indicates taking the first letters.
The initial letter of 'donkeys' is 'd'.
'air' placed around 'd' is 'adir'.
'n'+'adir'='NADIR'
(Other definitions for nadir that I've seen before include "Rock bottom" , "least successful point" , "You can't sink lower!" , "worst-case scenario?" , "Dinar (anag.)" .)