Computer suitable for use in large operation (6)
I believe the answer is:
laptop
'computer' is the definition.
(laptop is a part of computer)
'suitable for use in large operation' is the wordplay.
'suitable' becomes 'apt' (apt means appropriate or fitting).
'for use in' indicates putting letters inside.
'large' becomes 'L' (eg in clothes sizes).
'operation' becomes 'op'.
'l'+'op'='lop'
'apt' inserted into 'lop' is 'LAPTOP'.
(Other definitions for laptop that I've seen before include "Portable computer" , "Aid for businessman" , "device?" , "Kind of computer" , "Type of computer" .)