Mine gold initially in a lot of bedrock (4,1)

'mine' is the definition.
('bags I' informally means 'that's mine!')

'gold initially in a lot of bedrock' is the wordplay.
'initially' says to take the initial letters.
'in' is an insertion indicator.
'a lot of' means to remove the last letter (I've seen 'lots of' mean this (the majority of the word)).
'bedrock' becomes 'basis' (bedrock can mean a basis or foundation).
The first letter of 'gold' is 'g'.
'basis' with its final letter removed is 'basi'.
'g' placed inside 'basi' is 'BAGS I'.