Lineage’s length included in startling comment supported by daughter (5)
I believe the answer is:
blood
'lineage's' is the definition.
(synonyms)
'length included in startling comment supported by daughter' is the wordplay.
'length' becomes 'l' (abbreviation in geometry).
'included in' means one lot of letters goes inside another.
'startling comment' becomes 'boo' (I can't explain this - if you can you should give a lot more credence to this answer).
'supported by' says to put letters next to each other (in a down clue, some letters hold up others).
'daughter' becomes 'd' (genealogical abbreviation).
'l' going into 'boo' is 'bloo'.
'bloo'+'d'='BLOOD'
Can you help me to learn more?
(Other definitions for blood that I've seen before include "vital liquid" , "Kinship; temperament" , "Fluid in veins" , "Red fluid" , "Gore" .)