Fit one American up with item of furniture (8)
I believe the answer is:
suitable
'fit' is the definition.
('suitable' can be similar in meaning to 'fit')
'one american up with item of furniture' is the wordplay.
'one' becomes 'i' (Roman numeral).
'american' becomes 'us'.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'with' says to put letters next to each other.
'item of furniture' becomes 'table' (I've seen this before).
'i'+'us'='ius'
'ius' written backwards gives 'sui'.
'sui'+'table'='SUITABLE'
(Other definitions for suitable that I've seen before include "Appropriate, meet" , "Fitting, apt" , "In character" , "Satisfactory, worthy" , "Appropriate, apt" .)