Friendliness of retired toff, one in institution (8)
I believe the answer is:
bonhomie
'friendliness' is the definition.
(bonhomie is a kind of friendliness)
'retired toff one in institution' is the wordplay.
'retired' is a reversal indicator.
'toff' becomes 'nob' (UK slang for a wealthy or upper-class person).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'institution' becomes 'home' (home is a kind of institution**).
'nob' written backwards gives 'bon'.
'i' put inside 'home' is 'homie'.
'bon'+'homie'='BONHOMIE'
'of' is the link.
(Other definitions for bonhomie that I've seen before include "Warmth" , "genial nature" , "'Easy good nature, cheerful friendliness (8)'" , "Genial manner from hobo in me" , "good humour" .)