Pass round back of biggish group of trees (7)
I believe the answer is:
thicket
'group of trees' is the definition.
'pass round back of biggish' is the wordplay.
'pass' becomes 'ticket' (I've seen this before).
'round' means one lot of letters goes inside another.
'back of' says to take the final letters.
The last letter of 'biggish' is 'h'.
'ticket' going around 'h' is 'THICKET'.
(Other definitions for thicket that I've seen before include "Copse" , "Dense growth of bushes" , "Coppice" , "Dense shrubbery" , "dense vegetation" .)