Ancient creature's fertilizer held by one academic (9)
I believe the answer is:
iguanodon
'ancient creature's fertilizer held by one academic' is the definition.
I can't tell whether this definition defines the answer.
'fertilizer held by one academic' is the wordplay.
'fertilizer' becomes 'guano' (guano is a kind of fertilizer).
'held by' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'academic' becomes 'don' (type of academic - especially at Oxbridge).
'i'+'don'='idon'
'guano' going within 'idon' is 'IGUANODON'.
Can you help me to learn more?
(Other definitions for iguanodon that I've seen before include "Massive dinosaur with long heavy tail" , "reptile now extinct" , "Herbivorous dinosaur" , "Old lizard" , "In the past a massive herbivore" .)