Rh ultimately injected in him with our fluid (7)
I believe the answer is:
rhodium
'rh' is the definition.
(synonyms)
'ultimately injected in him with our fluid' is the wordplay.
'ultimately' suggests the final letters.
'in' means one lot of letters goes inside another.
'with' means one lot of letters go next to another.
'fluid' is an anagram indicator.
The last letter of 'injected' is 'd'.
'him'+'our'='himour'
'himour' with letters rearranged gives 'rhoium'.
'd' placed inside 'rhoium' is 'RHODIUM'.
(Other definitions for rhodium that I've seen before include "Hard metallic element" , "Hard metal" , "Chemical element, symbol Rh" , "Metallic element, atomic no 45" .)