Old park-keeper, Head of Yeovil Conservatory (8)
I believe the answer is:
orangery
'conservatory' is the definition.
(thesaurus)
'old park-keeper head of yeovil' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'park keeper' becomes 'ranger' (I've seen this before).
'head of' suggests taking the first letters.
The initial letter of 'yeovil' is 'y'.
'o'+'ranger'+'y'='ORANGERY'
(Other definitions for orangery that I've seen before include "fruit is grown here" , "greenhouse, maybe" , "where mandarins thrive" , "Place for growing fruit in cool climates" , "Greenhouse for fruit bushes" .)