Person finally squeezed into retro jeans, showing blubber (6)
I believe the answer is:
Here is my best explanation:
'blubber' is the definition.
('snivel' can be a synonym of 'blubber')
'person finally squeezed into retro jeans' is the subsidiary indication.
'finally' says to take the final letters.
'squeezed into' means one lot of letters goes inside another.
'retro' says the letters should be written in reverse.
'jeans' becomes 'levis' (I've seen this before).
The last letter of 'person' is 'n'.
'levis' backwards is 'sivel'.
'n' going 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", "Complain tearfully", "Blubber contemptibly", "Whine - sniff", "Cry in whining way, whinge", "Blub revoltingly", "Whimper" and "Whine (with a runny nose?)".)