Army’s building units designed to house exotic hens (6,3)
I believe the answer is:
nissen hut
'army's building' is the definition.
(steel structure used by the military)
'units designed to house exotic hens' is the wordplay.
'designed' indicates anagramming the letters (designed with the letters).
'to house' is an insertion indicator.
'exotic' indicates an anagram (an unusual form of the letters).
'units' is an anagram of 'nisut'.
'hens' anagrammed gives 'senh'.
'nisut' going around 'senh' is 'NISSEN HUT'.
(Other definitions for nissen hut that I've seen before include "Prefabricated corrugated iron dwelling" , "Corrugated iron shelter" , "Prefab" , "Prefabricated building of corrugated iron with a cement floor" , "Corrugated iron construction" .)