Fixed head of tool piercing smoothed wooden surface (7)
I believe the answer is:
planted
'fixed head of tool' is the definition.
'planted' can be an answer for 'fixed' (thesaurus). I am unsure of the 'head of tool' bit.
'piercing smoothed wooden surface' is the wordplay.
'piercing' is an insertion indicator (some letters pierce a hole in a word).
'smoothed' becomes 'planed' (I've seen this in another clue).
'wooden surface' becomes 't' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'planed' placed around 't' is 'PLANTED'.
Can you help me to learn more?
(Other definitions for planted that I've seen before include "Set in the soil for growth" , "Put in the ground" .)