Protective one’s maintaining alternative line (8)
I believe the answer is:
motherly
'protective' is the definition.
(I've seen this before)
'one's maintaining alternative line' is the wordplay.
'one's' becomes 'my' (both can mean belonging to the person writing).
'maintaining' means one lot of letters goes inside another (inserted letters are held or maintained).
'alternative' becomes 'other' (I've seen this before).
'line' becomes 'l' (used when specifying particular lines from a poem).
'other'+'l'='otherl'
'my' placed around 'otherl' is 'MOTHERLY'.
(Other definitions for motherly that I've seen before include "Maternal - gentle" , "Maternally protective" , "Nurturing and protective" , "Caring, protective" .)