is just the opposite of endocentric construction. If refers to a group of syntactically related words where none of the words is functionally equivalent to the group as a whole, that is, there is no definable “centre” or “head” inside the group. Exocentric construction usually includes basic sentence, prepositional phrase, predicate (verb+object) construction, and connective (be+complement) construction.
A morpheme, like a phoneme, is a linguistic abstraction, which must be realized as certain phonetic forms or variants in different phonetic environments.