Silly ass holding head of pin for pinheads (4)
I believe the answer is:
saps
'pinheads' is the definition.
(a foolish person)
'silly ass holding head of pin' is the wordplay.
'silly' indicates anagramming the letters.
'holding' indicates putting letters inside.
'head of' indicates taking the first letters.
The initial letter of 'pin' is 'p'.
'ass' with letters rearranged gives 'sas'.
'sas' placed around 'p' is 'SAPS'.
'for' is the link.
(Other definitions for saps that I've seen before include "More than one fool" , "trenches" , "mugs" , "Weakens, undermines" , "Vital juices" .)