Sex, it seems, makes a senior officer turn red (6)
I believe the answer is:
gender
'sex' is the definition.
(I know that sex can be written as gender)
'officer turn red' is the wordplay.
'officer' becomes 'gen' (abbreviation for general).
'turn' says the letters should be written in reverse.
'red' reversed gives 'der'.
'gen'+'der'='GENDER'
'it seems makes a senior' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for gender that I've seen before include "Class such as female or male" , "Masculine, feminine or neuter" , "Eg, male" , "Man or woman" , "Sexual status" .)