Slow up, being restricted by explosive chemical (7)
I believe the answer is:
hydrate
'chemical' is the definition.
(thesaurus)
'slow up being restricted by explosive' is the wordplay.
'slow' becomes 'tardy' (I've seen this before**).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'being restricted by' means one lot of letters goes inside another (I've seen this in other clues).
'explosive' becomes 'he' (abbreviation for high explosive).
'tardy' reversed gives 'ydrat'.
'ydrat' placed within 'he' is 'HYDRATE'.
(Other definitions for hydrate that I've seen before include "Give water" , "Apply water" , "Supply liquid" , "Moisturise" , "Thready (anag.)" .)