Minor test having intravenous injection (7)
I believe the answer is:
trivial
'minor' is the definition.
(I've seen this before)
'test having intravenous injection' is the wordplay.
'test' becomes 'trial' (synonyms).
'having' is an insertion indicator.
'intravenous injection' becomes 'iv'.
'trial' going around 'iv' is 'TRIVIAL'.
(Other definitions for trivial that I've seen before include "Rival it, being unimportant" , "Quite unimportant and paltry" , "Very easy" , "lacking weight" , "Piddling" .)