Island zoo initially invested in fat reptile (6)
I believe the answer is:
lizard
'reptile' is the definition.
(lizard is a kind of reptile)
'island zoo initially invested in fat' is the wordplay.
'initially' says to take the initial letters.
'invested in' is an insertion indicator.
'fat' becomes 'lard' (I've seen this before).
The first letters of 'island zoo' is 'iz'.
'iz' inserted within 'lard' is 'LIZARD'.
(Other definitions for lizard that I've seen before include "The most Southerly point of England" , "Gecko, chameleon or monitor?" , "Gecko, for example" , "Reptile with four legs and a tapering tail" , "Reptile with two pairs of legs" .)