In crimson, dead similar (7)
I believe the answer is:
related
'similar' is the definition.
(similar in meaning)
'in crimson dead' is the wordplay.
'in' indicates putting letters inside.
'crimson' becomes 'red' (synonyms).
'dead' becomes 'late' (similar in meaning).
'red' enclosing 'late' is 'RELATED'.
(Other definitions for related that I've seen before include "Told; linked" , "As siblings obviously are" , "Having family links" , "Like family members" , "of one family" .)