Choose to have an operation at the treatment centre (3)
I believe the answer is:
opt
'choose' is the definition.
(both can mean to pick something)
'an operation at the treatment centre' is the wordplay.
'an operation' becomes 'op'.
'at' says to put letters next to each other.
'centre' means to look at the middle letters.
The centre of 'treatment' is 't'.
'op'+'t'='OPT'
'to have' is the link.
(Other definitions for opt that I've seen before include "Elect" , "End or act of shutting" , "Select as a preferable alternative" , "Decide between possibilities, choose" , "Plump for" .)