A guardian takes pressure, that's obvious (8)
I believe the answer is:
apparent
'obvious' is the definition.
(similar in meaning)
'a guardian takes pressure' is the wordplay.
'guardian' becomes 'parent' ().
'takes' means one lot of letters goes inside another.
'pressure' becomes 'p' (scientific abbreviation).
'parent' enclosing 'p' is 'pparent'.
'a'+'pparent'='APPARENT'
'that's' is the link.
(Other definitions for apparent that I've seen before include "That may be seen" , "Evident or seeming to be" , "Clearly visible" , "readily understood" , "as such an heir" .)