In a reserved way, have a fling with heartless lady (5)
I believe the answer is:
shyly
'in a reserved way' is the definition.
(I've seen this in another clue)
'a fling with heartless lady' is the wordplay.
'a fling' becomes 'shy' (shy can mean to throw).
'with' says to put letters next to each other.
'heartless' suggests removing the centre.
'lady' with its middle taken out is 'ly'.
'shy'+'ly'='SHYLY'
'have' is the link.
(Other definitions for shyly that I've seen before include "In reserved way" , "In a timid manner" , "Nervously, timidly" .)