Tess upset with Christian getting back into synthetic hormones (8)
I believe the answer is:
steroids
'synthetic hormones' is the definition.
'steroids' can be an answer for 'hormones' (steroid is a kind of hormone). I'm not certain of the 'synthetic' bit.
'tess upset with christian getting back' is the wordplay.
'upset' indicates anagramming the letters.
'with' means one lot of letters goes inside another.
'christian' becomes 'dior' (fashion designer Christian Dior).
'getting back' is a reversal indicator.
'dior' back-to-front is 'roid'.
'tess' is an anagram of 'stes'.
'stes' going around 'roid' is 'STEROIDS'.
'into' acts as a link.
Can you help me to learn more?
(Other definitions for steroids that I've seen before include "Organic compounds, taken by some athletes for strength" , "Organic compounds taken by some athletes - do resist them" , "Synthetic hormones used illegally in sport" , "Body-builders" , "Sort side out with illegal drugs for athletes" .)