Inside sofas tend to tie up (6)
I believe the answer is:
fasten
'tie up' is the definition.
(I've seen this in another clue)
'inside sofas tend to' is the wordplay.
'inside' says the answer is hidden in the clue.
'FASTEN' is hidden in the letters of 'sofas tend to'.
(Other definitions for fasten that I've seen before include "Affix" , "Attach" , "Tie, secure" , "Fix to, secure" , "Shut" .)