Investigations involving lower part of reproductive glands (6)
I believe the answer is:
testes
'glands' is the definition.
(testis is a kind of gland)
'investigations involving lower part of reproductive' is the wordplay.
'investigations' becomes 'tests' (test can mean an investigation or inspection).
'involving' is an insertion indicator.
'lower part of' indicates one should take the final letters (in a down clue, the letter at the bottom).
The final letter of 'reproductive' is 'e'.
'tests' going around 'e' is 'TESTES'.
(Another definition for testes that I've seen is " balls".)