Principled old American curls up awkwardly at first (10)
I believe the answer is:
scrupulous
'principled' is the definition.
(similar in meaning)
'old american curls up awkwardly at first' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'american' becomes 'us' (abbreviation for United States).
'awkwardly' is an anagram indicator.
'at first' means one lot of letters go next to another.
'curls'+'up'='curlsup'
'curlsup' anagrammed gives 'scrupul'.
'o'+'us'='ous'
'ous' put after 'scrupul' is 'SCRUPULOUS'.
(Other definitions for scrupulous that I've seen before include "Very ethical" , "Conscientious, exact" , "Extremely conscientious" , "Moral" , "Conscientious, careful" .)