Prank in which he's causing less damage? (7)
I believe the answer is:
cheaper
'causing less damage?' is the definition.
(I've seen this before)
'prank in which he's' is the wordplay.
'prank' becomes 'caper' (synonyms).
'in which' means one lot of letters goes inside another.
'caper' placed around 'he' is 'CHEAPER'.
(Other definitions for cheaper that I've seen before include "causing less damage" , "Not as dear" , "more affordable" , "Comparatively despicable" , "More reasonable" .)