Began experiencing negative moods in teenage years, feeling active hostility (6)
I believe the answer is:
enmity
'feeling active hostility' is the definition.
'enmity' can be an answer for 'hostility' (synonyms). I am unsure of the 'feeling active' bit.
'began experiencing negative moods in teenage years' is the wordplay.
'began' suggests taking the first letters (I've seen 'begin' mean this).
'years' becomes 'y' (abbreviation).
The initial letters of 'experiencing negative moods in teenage' is 'enmit'.
'enmit'+'y'='ENMITY'
Can you help me to learn more?
(Other definitions for enmity that I've seen before include "Hostility between opposing people or countries" , "ill will" , "Hatred, hostility" , "needle" , "Animus" .)