Many a service involves needlework (4)
I believe the answer is:
'involves needlework' is the definition.
I can't judge whether this defines the answer.
'many a service' is the wordplay.
'many' becomes 'd' ().
'service' becomes 'RN' (abbreviation for Royal Navy).
Can you help me to learn more?
(Other definitions for darn that I've seen before include "Mend (e.g. socks)" , "Repair holes by sewing" , "Repair (sock)" , "Repair (with needle)" , "Mend (woollens)" .)