Valuable source I abandoned under pressure (8)
I believe the answer is:
precious
'valuable' is the definition.
(similar in meaning)
'source i abandoned under pressure' is the wordplay.
'abandoned' is an anagram indicator.
'under' says to put letters next to each other.
'pressure' becomes 'p' (abbreviation).
'source'+'i'='sourcei'
'sourcei' anagrammed gives 'recious'.
'recious' put after 'p' is 'PRECIOUS'.
(Other definitions for precious that I've seen before include "Cherished or valuable" , "highly esteemed" , "Valuable or dearly beloved" , "Very fastidious" , "Much loved" .)