English county: old US president donning new shirt here (13)
I believe the answer is:
hertfordshire
'old us president donning new shirt here' is the definition.
The definition suggests a singular noun which matches the answer.
'english county' is the wordplay.
'english' becomes 'hertford' (I am not sure about this - if you are sure you should believe this answer much more).
'county' becomes 'shire' (term for a county, eg Hampshire, Warwickshire etc).
'hertford'+'shire'='HERTFORDSHIRE'
Can you help me to learn more?
(Other definitions for hertfordshire that I've seen before include "Watford's county" , "Southern county" , "County of Watford and Ware" , "County to N. of London" , "A home county" .)