Crack is laughably centred on aviation concern (8)
I believe the answer is:
hairline
'crack' is the definition.
(I have seen 'Thin crack ' mean 'hairline' so perhaps 'crack' could also mean 'hairline')
'laughably centred on aviation concern' is the wordplay.
'centred' means to look at the middle letters.
'on' says to put letters next to each other.
'aviation' becomes 'air' (synonyms).
'concern' becomes 'line' (line is a kind of concern).
The middle letter of 'laughably' is 'h'.
'h'+'air'+'line'='HAIRLINE'
'is' is the link.
(Other definitions for hairline that I've seen before include "Very narrow opening" , "Of a crack, very thin" , "(Very fine) crack or fracture" , "A very narrow crack, a little hirsute?" , "Barnet's border" .)