Cricket jumper at the back of the lawn (11)
I believe the answer is:
grasshopper
'cricket' is the definition.
(thesaurus)
'jumper at the back of the lawn' is the wordplay.
'jumper' becomes 'hopper' (I've seen this before).
'at the back of' means one lot of letters go next to another.
'the lawn' becomes 'grass' (I've seen this before).
'hopper' after 'grass' is 'GRASSHOPPER'.
(Other definitions for grasshopper that I've seen before include "Excellent jumper" , "Insect (that won't stay still for long?)" , "Type of cricket" , "Jumping insect" , "six-footer with extra powerful legs" .)