Infectious disease giving same trouble to Les (7)
I believe the answer is:
measles
'infectious disease' is the definition.
(I've seen this before)
'giving same trouble to les' is the wordplay.
'giving' says to put letters next to each other.
'trouble' indicates an anagram.
'same' with letters rearranged gives 'meas'.
'meas' next to 'les' is 'MEASLES'.
(Other definitions for measles that I've seen before include "Children's viral disease" , "Viral disease with rash" , "Children's disease with red spots" , "Highly contagious viral disease" , "Viral disease once common in children" .)