One particular therapy primarily for a man (4)
I believe the answer is:
this
'one particular' is the definition.
(I've seen this before)
'therapy primarily for a man' is the wordplay.
'primarily' indicates taking the first letters.
'for' means one lot of letters go next to another.
'a man' becomes 'his' (I can't justify this - if you can you should believe this answer much more).
The first letter of 'therapy' is 't'.
't'+'his'='THIS'
(Other definitions for this that I've seen before include "Pronoun to point out nearby things" , "near thing" , "What's here" , "The thing under discussion" , "One particular" .)