Smart though not quiet — they're in the groove (5)
I believe the answer is:
styli
'they're in the groove' is the definition.
(plural of stylus, as in a record player stylus)
'smart though not quiet' is the wordplay.
'smart' becomes 'stylish' (smartly dressed).
'though not' is a deletion indicator.
'quiet' becomes 'sh' ('sh' is used to tell someone to be quiet).
'stylish' with 'sh' taken out is 'STYLI'.
(Other definitions for styli that I've seen before include "writing implements" , "LP record pickups" , "Writers of old" , "pens" , "Ancient writing instruments" .)