One who entertains head of automobile plant (5)
I believe the answer is:
hosta
'plant' is the definition.
The answer is an example.
'one who entertains head of automobile' is the wordplay.
'one who entertains' becomes 'host' (I've seen this before).
'head of' suggests taking the first letters.
The initial letter of 'automobile' is 'a'.
'host'+'a'='HOSTA'
(Other definitions for hosta that I've seen before include "Garden plant with ornamental foliage" , "Athos (anag) - perennial plant" , "Plant grown for foliage" , "Lily" , "flowering plant" .)