Bones in middle of trunk alongside seedy lane (5)
I believe the answer is:
ulnae
'bones' is the definition.
(ulna is a kind of bone)
'in middle of trunk alongside seedy lane' is the wordplay.
'in middle of' says to take the centre (I've seen 'in middle' mean this).
'alongside' means one lot of letters go next to another.
'seedy' is an anagram indicator (I've seen this in other clues).
The central letter of 'trunk' is 'u'.
'lane' is an anagram of 'lnae'.
'u'+'lnae'='ULNAE'
(Other definitions for ulnae that I've seen before include "Thin bones in upper arms" , "parts of limb" , "Army parts" .)