Resorts employing treatment, having nothing for drug at intervals (13)
I believe the answer is:
spasmodically
'at intervals' is the definition.
(thesaurus)
'resorts employing treatment having nothing for drug' is the wordplay.
I cannot really understand how this works, but
'resorts' could be 'spas' (spa is a kind of resort) and 'spas' is located in the answer.
'nothing' could be 'o' (looks like zero - 0) and 'o' is found in the answer.
'drug' could be 'c' (C is an example) and 'c' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for spasmodically that I've seen before include "In irregular bursts" , "In fits and starts" .)