A tick for one old king skipping posh old school (9)
I believe the answer is:
arthropod
'a tick for one' is the definition.
(tick is a kind of arthropod)
'old king skipping posh old school' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found within the answer.
'king' could be 'r' (r is abbreviation for rex) and 'r' is present in the answer.
'school' could be 'pod' (I've seen this before) and 'pod' is found in the answer.
This explanation may well be incorrect...
(Other definitions for arthropod that I've seen before include "One has external skeleton" , "spider, for example" , "Such as a crustacean or spider" , "worker, perhaps" , "Invertebrate" .)