Surprisingly neat once holding puss’s string together (11)
I believe the answer is:
concatenate
'string together' is the definition.
(I've seen this before)
'surprisingly neat once holding puss's' is the wordplay.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
'holding' means one lot of letters goes inside another.
'puss' becomes 'cat' (I've seen this in other clues).
'neat'+'once'='neatonce'
'neatonce' is an anagram of 'conenate'.
'conenate' placed around 'cat' is 'CONCATENATE'.
(Other definitions for concatenate that I've seen before include "Link together in a chain" , "String together" .)