Sid hides his appreciation of being sober (5)
I believe the answer is:
staid
'sober' is the definition.
(synonyms)
'sid hides his appreciation' is the wordplay.
'hides' indicates putting letters inside.
'his appreciation' becomes 'ta' (I can't explain this - if you can you should give a lot more credence to this answer).
'sid' going around 'ta' is 'STAID'.
'of being' acts as a link.
Can you help me to learn more?
(Other definitions for staid that I've seen before include "Settled" , "Serious or sober in character" , "Restrained (in character)" , "Calm, respectable and unadventurous" , "Sedate, decorous" .)