Person finally squeezed into retro jeans, showing blubber (6)
I believe the answer is:
snivel
'blubber' is the definition.
(I've seen this before)
'person finally squeezed into retro jeans' is the wordplay.
'finally' indicates one should take the final letters.
'squeezed into' is an insertion indicator.
'retro' says the letters should be written in reverse (retro is Latin for 'backwards').
'jeans' becomes 'levis' (I've seen this before).
The final letter of 'person' is 'n'.
'levis' in reverse letter order is 'sivel'.
'n' inserted inside 'sivel' is 'SNIVEL'.
'showing' is the link.
(Other definitions for snivel that I've seen before include "Cry in miserable whining way" , "Sniff - whine" , "moan" , "Blub revoltingly" , "Complain whiningly" .)