Fix hem with no top on the sack (5)
I believe the answer is:
embed
'fix' is the definition.
(I've seen this before)
'hem with no top on the sack' is the wordplay.
'with no top on' means to remove the first letter (the first or top letter is removed).
'the sack' becomes 'bed' ('in the sack' means 'in bed').
'hem' with its first letter removed is 'em'.
'em'+'bed'='EMBED'
(Other definitions for embed that I've seen before include "the opposite of 19 [DEBUNK]?" , "one placed in a military unit" , "Plant deeply" , "Plant firmly" , "Fix firmly in surrounding mass" .)