Heading off unwell during test is crafty (6)
I believe the answer is:
tricky
'crafty' is the definition.
(I know that crafty can be written as tricky)
'heading off unwell during test' is the wordplay.
'heading off' means to remove the first letter.
'unwell' becomes 'sick' (similar in meaning).
'during' means one lot of letters goes inside another.
'test' becomes 'try' (synonyms).
'sick' with its initial letter taken away is 'ick'.
'ick' put inside 'try' is 'TRICKY'.
'is' is the link.
(Other definitions for tricky that I've seen before include "Crafty, cunnning" , "Deceitful, crafty" , "Involving snags" , "Problematic - deceitful" , "Difficult; sly" .)