You dont get cross when you get one (4)
I believe the answer is:
tick
'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'you dont get cross when you' is the wordplay.
I cannot really understand how this works, but
'cross' could be 't' (shape) and 't' is found within the answer.
The remaining letters 'ick' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'get' acts as a link.
Can you help me to learn more?
(Other definitions for tick that I've seen before include "Credit; insect" , "Mark indicating correctness" , "Blood-sucking mite or parasitic insect" , "Mark of work done (right)" , "Credit - bloodsucker" .)