Gaps make it hard to fix sole (5)
I believe the answer is:
holes
'gaps' is the definition.
(I know this)
'hard to fix sole' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to fix' indicates anagramming the letters.
'sole' is an anagram of 'oles'.
'h'+'oles'='HOLES'
'make it' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for holes that I've seen before include "Cavities" , "Openings" , "Apertures" , "Empty spaces" , "Scrapes" .)