Interchangeable part (6)
I believe the answer is:
module
'interchangeable part' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for module that I've seen before include "Element of course etc." , "Unit of a study course" , "Short study course" , "Self-contained unit, of study perhaps" , "Complete part (of spacecraft or course)" .)