Overcrowded relationships lasting? Er … unlikely (9)
I believe the answer is:
triangles
'overcrowded relationships' is the definition.
(as in love triangles)
'lasting? er unlikely' is the wordplay.
'unlikely' is an anagram indicator.
'lasting'+'er'='lastinger'
'lastinger' anagrammed gives 'TRIANGLES'.
(Other definitions for triangles that I've seen before include "Musical instruments" , "Percussion instruments" , "Three-sided shapes" , "Three-sided figures" , "Geometrical figures" .)