In the main got lost, somehow, in my keeping (6)
I believe the answer is:
mostly
'in the main' is the definition.
(I've seen this in another clue)
'in my keeping' is the wordplay.
'in' is an insertion indicator.
'keeping' becomes 'ostl' (I can't justify this - if you can you should give a lot more credence to this answer).
'my' going around 'ostl' is 'MOSTLY'.
'got lost somehow' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for mostly that I've seen before include "In the main, nearly all" , "More often than not" , "On the whole" , "Largely" , "In large part" .)