One shelter in which to make money seriously (2,7)
I believe the answer is:
in earnest
'seriously' is the definition.
(I've seen this before)
'one shelter in which to make money' is the wordplay.
'one' becomes 'i' (Roman numeral).
'shelter' becomes 'nest' (I've seen this in other clues).
'in which' is an insertion indicator.
'to make money' becomes 'earn' (I've seen this before).
'nest' going around 'earn' is 'nearnest'.
'i'+'nearnest'='IN EARNEST'
(Other definitions for in earnest that I've seen before include "Purposefully" , "Sincere and serious" , "Not joking" , "with serious intent" , "With serious intention, not fooling about (2.7)" .)