First of early NHS units built in military accommodation (6,3)
I believe the answer is:
nissen hut
'military accommodation' is the definition.
(type of structure for military use)
'first of early nhs units built' is the wordplay.
'first of early' becomes 'e' (1st letter of 'early').
'built' indicates anagramming the letters.
'e'+'nhs'+'units'='enhsunits'
'enhsunits' anagrammed gives 'NISSEN HUT'.
'in' is the link.
(Other definitions for nissen hut that I've seen before include "army building" , "Corrugated iron construction" , "spartan accommodation?" , "Military shelter made of corrugated steel" , "Prefabricated corrugated iron dwelling" .)