Determine fortnightly magazine is a swindle (4,3,4)
I believe the answer is:
find the lady
'a swindle' is the definition.
(card trick)
'determine fortnightly magazine' is the wordplay.
'determine' becomes 'find' ('find' can be a synonym of 'determine').
'fortnightly magazine' becomes 'thelady' (UK magazine).
'find'+'thelady'='FIND THE LADY'
'is' acts as a link.
(Other definitions for find the lady that I've seen before include "Three-card trick" , "Cherchez la femme" , "there's a trick to it" .)