'Mother', one ace opener for Lennon work (8)
I believe the answer is:
material
'work' is the definition.
(eg material by a writer)
'mother one ace opener for lennon' is the wordplay.
'mother' becomes 'mater' (old-fashioned term for mother).
'one' becomes 'i' (Roman numeral).
'ace' becomes 'a'.
'opener for' indicates taking the first letters.
The first letter of 'lennon' is 'l'.
'mater'+'i'+'a'+'l'='MATERIAL'
(Other definitions for material that I've seen before include "Essential" , "of serious importance" , "Felt, perhaps" , "Important; fabric" , "Relevant sort of cloth?" .)