Hold back and tie in a knot (6)
I believe the answer is:
detain
'hold back' is the definition.
(I've seen this before)
'and tie in a knot' is the wordplay.
'in a knot' indicates anagramming the letters.
'and'+'tie'='andtie'
'andtie' is an anagram of 'DETAIN'.
(Other definitions for detain that I've seen before include "Hold (someone) back" , "Hang on to" , "Keep waiting or in confinement" , "Hold in custody or delay" , "Delay or take into custody" .)