Determined to hide kiss in explicit message (4)
I believe the answer is:
sext
'explicit message' is the definition.
(I've seen this before)
'determined to hide kiss' is the wordplay.
'determined' becomes 'set' (similar in meaning).
'to hide' indicates putting letters inside.
'kiss' becomes 'x'.
'set' going around 'x' is 'SEXT'.
'in' acts as a link.
(Other definitions for sext that I've seen before include "Explicit message" , "service" .)