Zoo trips occasionally lift company (4)
I believe the answer is:
otis
'lift company' is the definition.
(Otis is an elevator manufacturer)
'zoo trips occasionally' is the wordplay.
'occasionally' indicates alternate letters.
'zoo'+'trips'='zootrips'
The alternating letters of 'zootrips' are 'OTIS'.
(Other definitions for otis that I've seen before include "Company providing lifts" , "Place of worship; side part of head" , "lift company" , "Regretful miss" , "---- Redding, 1941-67, soul singer" .)