One you knit soundly? (4)
I believe the answer is:
unit
'one' is the definition.
(synonyms)
'you knit soundly' is the wordplay.
'you' becomes 'u' (commonly used in online communication).
'soundly' indicates a 'sounds like' (homophone) clue.
'knit' is a homophone of 'nit'.
'u'+'nit'='UNIT'
(Other definitions for unit that I've seen before include "Small military detachment" , "One - kitchen cabinet" , "A considerable number of soldiers" , "1 entity" , "Individual thing or group regarded as complete" .)