Sid hides his appreciation of being sober (5)
I believe the answer is:
staid
'being sober' is the definition.
('staid' can be a synonym of 'sober')
'sid hides his appreciation' is the wordplay.
'hides' means one lot of letters goes inside another.
'his appreciation' becomes 'ta' (I can't explain this - if you can you should believe this answer much more).
'sid' placed around 'ta' is 'STAID'.
'of' is the 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" .)