Silly head of union in danger before end of strike (7)
I believe the answer is:
puerile
'silly' is the definition.
(I've seen this before)
'head of union in danger before end of strike' is the wordplay.
'head of' suggests taking the first letters.
'in' is an insertion indicator.
'danger' becomes 'peril' (peril is a kind of danger).
'before' says to put letters next to each other.
'end of' indicates one should take the final letters.
The first letter of 'union' is 'u'.
The last letter of 'strike' is 'e'.
'u' placed inside 'peril' is 'pueril'.
'pueril'+'e'='PUERILE'
(Other definitions for puerile that I've seen before include "Childish, immature" , "Childishly silly" , "Babyish" , "Childish, silly, trivial" , "Childishly stupid" .)