Stop when lad with new stilts takes a tumble (10)
I believe the answer is:
standstill
'stop' is the definition.
(standstill is a kind of stop)
'lad with new stilts takes a tumble' is the wordplay.
'with' says to put letters next to each other.
'new' becomes 'n'.
'takes a tumble' indicates anagramming the letters.
'lad'+'n'+'stilts'='ladnstilts'
'ladnstilts' is an anagram of 'STANDSTILL'.
'when' acts as a link.
(Other definitions for standstill that I've seen before include "Perhaps jam" , "lack of movement" , "negation of motion" , "Complete stop" , "stoppage" .)