Mischievous people affected to board ship (6)
I believe the answer is:
scamps
'mischievous people' is the definition.
'affected to board ship' is the wordplay.
'affected' becomes 'camp' (I've seen this before).
'to board' means one lot of letters goes inside another.
'ship' becomes 'ss'.
'camp' placed within 'ss' is 'SCAMPS'.
(Other definitions for scamps that I've seen before include "Little rascals, mischievous children" , "Scallywags" , "Naughty little rascals" , "Mischevious young children" , "'Rascals, imps (6)'" .)