Personnel department in company is pleasant but brief though it's becoming serious (7)
I believe the answer is:
chronic
'brief though it's becoming serious' is the definition.
I can't judge whether this defines the answer.
'personnel department in company is pleasant' is the wordplay.
'personnel department' becomes 'HR' (Human Resources).
'in' means one lot of letters goes inside another.
'company' becomes 'co' (abbreviation).
'is pleasant' becomes 'nic' (I can't justify this - if you can you should believe this answer much more).
'hr' placed inside 'co' is 'chro'.
'chro'+'nic'='CHRONIC'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for chronic that I've seen before include "Of illness, prolonged or constantly recurring" , "Awful" , "Deep-seated - long-lasting" , "(Of disease) long-lasting" , "Not acute" .)