Father joins retired teacher in European capital (5)
I believe the answer is:
paris
'european capital' is the definition.
(I've seen this before)
'father joins retired teacher' is the wordplay.
'father' becomes 'pa' (pa is a kind of father).
'joins' says to put letters next to each other.
'retired' says the letters should be written backwards.
'teacher' becomes 'sir' (I've seen this before**).
'sir' written backwards gives 'ris'.
'pa'+'ris'='PARIS'
'in' acts as a link.
(Other definitions for paris that I've seen before include "'I Love - - ', song by 14" , "captivated Helen" , "English chronicler" , "Helen's abductor" , "French capital - Trojan prince" .)