Declare invalid having to check blood type yearly (6)
I believe the answer is:
annual
'yearly' is the definition.
(synonyms)
'declare invalid having to check blood' is the wordplay.
'declare invalid' becomes 'annul' (I've seen this before).
'having' is an insertion indicator.
'to check blood' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'annul' going around 'a' is 'ANNUAL'.
'type' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for annual that I've seen before include "Regular publication" , "Plant that lives and dies within one year" , "Twelve-monthly" , "Occurring yearly" , "Occuring once a year" .)