Cross when back legs start to get weary (7)
I believe the answer is:
saltire
'cross' is the definition.
(saltire is a kind of cross)
'when back legs start to get weary' is the wordplay.
'when' becomes 'as'.
'back' says the letters should be written in reverse.
'start to' indicates taking the first letters.
'get weary' becomes 'tire' (I've seen this before).
The initial letter of 'legs' is 'l'.
'as' back-to-front is 'sa'.
'sa'+'l'+'tire'='SALTIRE'
(Other definitions for saltire that I've seen before include "X-shaped cross" , "Cross like St Andrew's" , "Scottish cross" , "standard for Scots" , "flag - 8's? [SAINT ANDREW]" .)