Pick up source of tremendous courage (5)
I believe the answer is:
heart
'courage' is the definition.
(heart is a kind of courage)
'pick up source of tremendous' is the wordplay.
'pick up' becomes 'hear' (synonyms).
'source of' indicates taking the first letters (the start - or source - of the word).
The first letter of 'tremendous' is 't'.
'hear'+'t'='HEART'
(Other definitions for heart that I've seen before include "Muscular organ - compassion" , "Playing card - pity" , "one in suit" , "Muscular organ for blood circulation" , "Pumping organ" .)