Start to cross leg, perhaps — shin up? (5)
I believe the answer is:
climb
'shin up?' is the definition.
(thesaurus)
'start to cross leg' is the wordplay.
'start to' indicates taking the first letters.
'leg' becomes 'limb' (I've seen this before).
The initial letter of 'cross' is 'c'.
'c'+'limb'='CLIMB'
'perhaps' is the link.
(Other definitions for climb that I've seen before include "Slope upwards" , "showing the way up" , "Rise, mount" , "Rise to a higher level" , "Go upwards" .)