Think much of present-day source of mineral (5)
I believe the answer is:
adore
'think much of' is the definition.
(I've seen this in another clue)
'present-day source of mineral' is the wordplay.
'present day' becomes 'AD' (as opposed to BC).
'source of mineral' becomes 'ore' (I've seen this before).
'ad'+'ore'='ADORE'
(Other definitions for adore that I've seen before include "Worship and glorify" , "Worship, venerate" , "Love greatly" , "Like a lot" , "Love, worship" .)