Group holding start of this instruction to printer (4)
I believe the answer is:
stet
'instruction to printer' is the definition.
(I've seen this before)
'group holding start of this' is the wordplay.
'group' becomes 'set' (set is a kind of group**).
'holding' is an insertion indicator.
'start of' suggests taking the first letters.
The first letter of 'this' is 't'.
'set' placed around 't' is 'STET'.
(Other definitions for stet that I've seen before include "Print as is, don't alter spelling" , "Editorial instruction meaning "let it stand"" , "marginal instruction" , "Ignore crossing" , "indicates second thoughts are not always better" .)